Apr 5, 2023
Apr 5, 2023

Underdog Betting: Why bet on the underdog in an ATP Masters 1000 tournament?

Underdog Betting: Why bet on the underdog in an ATP Masters 1000 tournament?

In my first Pinnacle Sports article, I analysed the performance of men’s tennis underdogs by comparing Grand Slam performance against other ATP tournaments.

The conclusion was that, overall, it was a disaster to bet on the underdogs. We saw that the favourite-longshot bias was much more prevalent in Grand Slam matches than non-Grand Slam matches.

Some of the reasons we adduced were: a lower demand elasticity for underdogs in Grand Slams than in non-Grand Slams by bettors, bettors not fully evaluating the impact of the “best-of-five sets factor”, and/or bookmakers being more risk averse on longshots due to an increased risk.

In this article I have conducted the same exercise for the ATP Masters 1000 events.

Underdog Betting: Analysis and Results

To complete this analysis, I have gathered the Pinnacle closing prices of all the main draw matches played from 2010 to the end of 2022. The 2023 data is not included.

After cleaning the data (due to incorrect and missing odds), and taking out withdrawals and retirements when the first set was not completed, I came up with a total of 32,419 fixtures from Masters 1000, ATP 250, ATP 500, and Grand Slam tournaments.

Once I had the data, I hosted a poll on my Twitter account. I asked “In which type of tournaments do you think underdogs do it better (= favourites do it worse)?”.


Most (58.8%) chose “ATP 250”. You can also observe that the higher the importance of the tournament, the less answers in favour of underdogs performing better.

The rationale behind these answers is simple. It is logical to assume that the ATP 250 tournaments should be where the underdogs perform better.

The motivation and desire to battle the best players could be lower, in contrast to Grand Slams, where motivation of the players is at its peak.

However, the 58.8% that chose ATP 250 tournaments were in fact wrong, and only 16% of the responses were correct. Here are the real results:


We can see here that the Masters 1000 tournaments are clearly the type of tournaments where the underdogs perform better.

The size of the samples is big enough that we can compare the different yields without any statistical constraints. If you had bet blindly on all the underdogs in every Masters 1000 match for the last 13 seasons, you would have only suffered a negative 1.2% yield.

This is a real surprise if we take into account:

  • That the overall negative yield if you had bet on all the underdogs is -6.1%, nearly 5 points lower
  • That Masters 1000 tournaments are the second most important tournaments after Grand Slams, so the motivation of players is usually very high
  • The favourite-longshot bias

The favourite-longshot bias is the tendency to undervalue favourites and overvalue “longshots” in sports betting.

On average, odds offered at higher prices have a higher margin 

On average, the odds offered by bookmakers at higher prices have a higher margin, and vice versa. Moreover, from certain levels, the higher the odds, the higher the margin.

This means that, for a bettor, the likelihood of winning when betting on longshots is lower, as a larger margin for the bookmaker translates into a lower expected yield.

We can see that the average odds of underdogs in the Masters 1000 tournaments (4.40) are practically identical to that of the ATP 500 (4.38). However, the difference in yields is 3.7 points (-4.9% vs -1.2%).

But comparing them to the ATP 250 tournaments is even more remarkable, where the average odds for the underdogs are 3.56. However, the yield is 3.60 points lower (-4.8% vs -1.2%).

But why, when it should be the opposite? According to the favourite-longshot bias, the higher the odds, the higher the negative yield.

But the average price for Masters 1000 underdogs is noticeably higher than that of the ATP 250s, and the yield is much less negative.

It seems that the favourite-longshot bias is not applicable here. And the reason is not that the sample is too low - we have 14,122 ATP 250 matches and 6694 Masters 1000 games, which is large enough to dismiss any sample issues.

Let’s also consider that the courts of these events are both clay and hard, so we cannot affirm that the reason is linked to any specific kind of surface.

Why underdogs perform in Masters 1000 tournaments

So now it’s time to try to explain the strong performance of the underdogs (and weak performance of the favourites) in the Masters 1000 tournaments.

In my opinion, the most likely explanation is that bettors overestimate the “motivation factor” in tennis.

Bettors think that, ceteris paribus, the motivation of a player is closely linked to their results.

This is then reflected in the market driving down the prices of the favourites whilst raising the prices of the underdogs to levels that, on average, make the odds for the favourites bad value.

The overall yield of favourites in the Masters 1000 tournaments is -2.8%, -2.1% for the ATP 250s, -2.0% for the ATP 500s, and +0.1% for the Grand Slams.

Of course, you won’t win in the long run betting only on the underdogs in the Masters 1000.

However, if we consider how tough it is to win when betting on the underdogs overall, it seems that the Masters 1000 is clearly the best option to do it.

Some might say that in the Grand Slams, where motivation is at its highest, this doesn’t happen.

In fact, it’s actually the opposite, as clearly the favourites perform much better than the underdogs, whose performance is often a disaster. However, we should consider that the best-of-five sets factor clearly undermines this comparison.

In summary, we have seen that Masters 1000 tournaments are clearly the most favourable to the underdogs.

If you are an underdog bettor in tennis like myself, and you have been betting in this fashion for years, you will have probably achieved your best results in this type of tournament.

The reasons for such strange statistics are open to debate, but in my opinion, bettors have a bias that overvalues the chances of the favourites, and undervalues that of the outliers.

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