Yo I’m super curious about this.
The majority of my tennis live-bets (I will go over exceptions at the end) fall into the following category;
Identify the clearly better player, and when he/she faces some adversity they can overcome, bet them at a much better price than their pre-match odds.
Sounds really simple, right? Actually, it's not. It's complicated as hell, and I've been refining my approach for months. Seemingly straightforward, general terms like "clearly better" actually become very complex when you try to judge them concretely. Let's look at the three elements of that little sentence and what goes into each one;
1. A clearly better player
What we're looking for is a considerable, measurable disparity in player skill. This is, as usual, a lot easier in the women's game than the men's. In the men's game, even players ranked 150th in the world can be extremely dangerous and play great tennis for stretches of time. A player ranked 100th can absolutely upset the 10th ranked player, without the latter even performing that poorly on the day.
In the women's game, such results are a lot rarer, and the 150th best player freaking sucks.
Determining player skill is a mix of looking up match results (and potentially stats, although I don't incorporate that yet) and looking at the quality of play. I mentioned rankings, but they're only a very, very rough approximation and not particularly useful. Especially with how wonky they are in the pandemic era, with temporary new rules allowing results from 2019 to be counted, and the unusually small number of tournaments in 2020, you're better looking at recent results.
When we look at match results, we're trying to get a sense of a player's typical performance, whether they're an older, declining player losing a bunch of a matches and going down the ladder, or on the flipside, a young, improving player rising up the ranks.
One very important factor to consider too is playing surface, specifically clay or hard courts. (Carpets can be treated similar to hard courts and grass is really rare nowadays) This isn't as huge a distinction as it would have been 20 years ago, since they've slowed down hard courts so much, but it's still important.
Christian Garin might be ranked 22nd in the world, but on hard courts, he's probably not even in the world's top 100. On clay courts, however, he is a top 10 player. Thus, on hard courts, for instance, it's possible that someone ranked 50th will be the clearly better player over Garin.
On top of that, you really want to observe the players and what they do well and poorly, their various attributes, etc. I can't really give too much general advice here; I used to play tennis, so I know what to look for, and the sport is a HELL of a lot simpler than MMA. I've started a list of notes similar to what I have for MMA and am constantly editing and adding to it.
One other important factor to consider is fatigue. How much tennis has each person played? Oftentimes, that may lead to a disparity that wouldn't otherwise be there. For instance, Dominic Thiem would normally be favored against Diego Schwartzman, even on clay. However, at last year's French Open, when they met in the quarterfinal, Thiem was absolutely exhausted late in the 4th set, a product of not only that match, but the tough matches he had previously in the tournament AND all the tennis and emotion he had exerted at the US Open he had recently won. He could still beat a ton of guys simply running on fumes, but not a legitimate top-10 talent with tremendous conditioning like Schwartzman, who won the 4th set tie-breaker and then destroyed him in the 5th and final set.
Keep in mind too that the oddsmakers aren't completely dumb. Jannik Sinner is the best young prospect in tennis and while he is ranked 36th, the oddsmakers treat him like a borderline top 10. And honestly, they're probably not wrong to do so. Similarly, Sebastian Korda is the second-best young prospect in tennis and despite being ranked 119th at Delray Beach, was at even odds against 35th-ranked Huber Hurkacz in the finals. Considering he had beaten the 52th, 25th, and 74th best guys in the world to get there, this was also justified.
2. Adversity they can overcome
Firstly, we have to consider what kind of deficit can reasonably be overcome, and secondly, what was the cause of this deficit to begin with?
Adversity I really love to see is an early break in the first set. Maybe the better player hasn't warmed up yet. Maybe he hasn't adjusted to his opponent's game. Maybe he lost it on purpose because he is point-shaving and/or partially fixing the match. (There is a TON of this in tennis, which is why I positively refuse to ever pre-bet Challengers and ITF matches and will only live-bet them)
Whatever the reason, oftentimes this might afford a great opportunity, depending on the numbers.
Of course, a weaker player simply keeping pace deep into the 1st set and/or getting a break opportunity or two can cause the odds to improve a lot. For instance, I mentioned betting Osaka against a very tired Begu (three straight matches that came down to the wire in the 3rd set) at -286 when she was about -1000 pre-match. All that was required was for the 1st set to be 4-4, with Osaka serving at 15-40. Now sure, Begu had 2 break point chances and would have served for the set, but the odds were still too good to pass up considering how clearly better Osaka is and how woefully tired Begu was. (which manifested itself later in the 1st set and a 2nd set she lost 6-1)
Even dropping the first set might not be too big a deal. Osaka lost the first set to Katie Boulter, ranked 371st in the world. Now, Boulter is better than that rating, as she was 85th prior to an injury that caused an absence late in 2019 (if you don't play tournaments your ranking drops), and had beaten the 47th-ranked Coco Gauff in a tense 3-setter a round earlier.
But seriously, come on. Osaka should wipe the fucking floor with Boulter. The difference in power alone is laughable. Thus, Osaka being something like -150 after dropping the first set was still great odds, despite how crazy it looked.
Of course, this isn't fool-proof. Earlier in the tournament, Sabalenka lost the first to Kaia Kanepi, ranked 93rd in the world. Now, Kanepi used to be as high as 15th in the world, but that was almost a whole decade ago, and she is now 35 years old and plagued with injuries. Plus, Sabalenka dropping sets badly happens from time to time and normally she recovers easily to crush the opponent. So getting her at +100 felt like a gift.
And indeed, Sabalenka crushed Kanepi 6-2 in the 2nd set and was something like a -550 favorite to start the 3rd. There, however, Kanepi played one of the best sets of tennis in her entire career. Sabalenka didn't even play that bad, but Kanepi just demolished her 6-1. Honestly, I doubt more than 5 female tennis players, and possibly far less than that could have beaten Kanepi with their average level of 3rd set play. These things happen!
Also, when the "weaker" player is up a set and a break in a best of 3, it's time to give it up. Yes, occasionally you will get spectacular comebacks. For instance, Sofia Kenin was down a set and a break to Jessica Pegula and was something like +1000 at that point, and went on to record an incredible comeback victory, but you're going to lose 15 such bets for every one that you win.
One thing that helps on deciding whether to bet is how the players got there. Ideally, you want it to be the better player performing below their level AND the weaker player playing out of their mind. That way, if only one regresses towards the mean, you get the result you want. Otherwise, if it's just the better player being awful, then it's possible the weaker player can raise their level. And if it's the weaker player being out of their mind, what if the better player has a poor set?
Also, you want to look at things like unsustainable success and hitting tons of low-percentage shots.
If a player is going for crazy cross-court forehand winners into the service box and keeps hitting them time and again, that's a great sign. There's just no way to do so consistently. If they're getting in 80% of their first serves, another great sign, as that's also an unsustainable percentage.
On the flipside, it also helps to avoid betting against certain "weaker" players. Ekaterina Alexandrova is 33rd in the world and was recently facing the world #2, Simona Halep. I had little desire to bet on Halep after Alexandrova won the 1st because she is a ridiculously high-variance player, going for crazy winners on evey point, but is also fully capable of keeping this up for the duration of a full match. Of course, she struggles to do it for the duration of a tournament, but when she is on, she can blow much higher-rated opponents off the court, which is exactly what happened to Halep.
3. Much better price
This really depends on the situation in question and the disparity in skill. In general, I avoid anything that isn't -500 or better, but really, -2XX to small plus odds is the sweet spot. A quick bit of arithmetic will tell you that if two evenly matched opponents are playing, and A wins set 1, then B has 1/2*1/2 =1/4 chance of winning the next two and the match, or +300. The significance of breaks is more tricky and depends on what point in the set they occur and how good each player serves and returns.
Other types of tennis live-bets-
1. Over-hyped "name" players.
Players who, due to fame or past great accomplishments, get way more respect than they deserve. Serena Williams nowadays being an example. Andy Murray when he was initially coming off hip replacement surgery was another example. Coco Gauff is presently a little bit of an example, but not as prominent.
These are rare cases, since few players become so hyped, and most deserve it (I mentioned Sinner and Korda above, and Djokovic and Nadal are STILL worthy of their lofty odds), but they're good opportunities for a while, until people catch on.
2. Very evenly-matched players where one guy is +120 or more for inexplicable reasons.
More a pre-bet, but it occasionally seeps into live-bets, too. Essentially, most tennis capping I've read online is really bad, and a lot of their "reasoning" largely irrelevant to the results of a tennis match. Yet, it effects odds from players and oddsmakers alike. If you're scratching your head about why the +120 player isn't at least evens and know the sport, trust your read and play him/her.
3. Weaker player is close in skill, is up a set, or up a set down a break, beginning of the 3rd, beginning of the tiebreaker, but isn't properly respected.
Maybe, as you watch the match, you realize the two players are actually very close in skill. Not exactly equal, but far more level than you thought. Yet, the live odds don't reflect this at all. Maybe, when the weaker player has won a set and is down a break early in the second, he is still available at plus numbers. Definitely worth playing them in such cases.
This becomes even better if it's early in set 3 and either both players or the better player are clearly fatigued. It becomes very close to a 50/50 then, but that's not always reflected. Tie-breaks can be fun, too.
A great example of this, which I whiffed on betting because I was focused on the UFC card, was the match between Jennifer Brady and Ann Li. Li was playing on a very close level to the better Brady, with Li winning the 1st set in a tie-breaker, 7-6, and Brady winning the 2nd in a tie-breaker, 6-7. It doesn't get any closer!
Yet, reflecting Brady's pre-match odds of something like -280, the deciding tie-break of first to 10 points had Brady as -200 to Li's +150! Sure, Brady is maybe a little bit better, but how prominent is that even going to be over so few points, when we've seen such parity over so many points?
Li at +150 would have been a great play, and I sure wish I had made it.
Anywho, there are a lot more details, but hope this helps.