58%
The sum of all the probabilities has to be 100%. In tennis, because there's no time limit, a match will always result in a win or a loss. Since Garry has won 42% of his games, that means that he's lost 100% - 42% = 58% of his games. And if you assume his win/loss ratio is reflective of his future games as well as his past games, then he as a 58% chances of loosing his next match.
