The result of a games handicap in Tennis is calculated by adding up all games won per player and then applying the handicap.
For example, if player A beats player B 7-6 6-7 7-6, Player A would have won 20 games to 19. A bet with a handicap of -0.5 on player A would win as 20-0.5(=19.5) is more than 19.
Player A can win the match and still lose the -0.5 handicap. If player A vs player B ends 7-6 2-6 7-6 for example, the total games won by player A would be 16, with player B winning 18 games.
16-0.5 is less than 18 so a -0.5 handicap bet on player A would lose.