Over Under betting is available on a number of tennis betting odds, most commonly the total games market. The sportsbook will give you a number of games (for example 35.5), and it will be up to you to determine if there will be less or more throughout the match.
Let’s use last year’s French Open men’s final as an example. Rafael Nadal was the firm favourite against Dominic Thiem, although the line was set at 35.5 games. This means that the sportsbooks were counting on three very tight sets, or potentially the match going to a fourth set, even though it seemed reasonable to predict a straight-sets victory to Novak given the form he has been in.
The match ended as such, 6-4, 6-3, 6-2, making for a total of 27 games – well under the line. A simple unders bet would have fetched you around -112, but if you had picked your own line and chosen under +29.5 games, for example, you would have taken home winnings at 4.20.