Match winner and total sets
Pick who wins the match. In a best-of-three, the winner takes two sets, so the match lasts two or three sets. Total sets bets on exactly that: under 2.5 sets wins on a straight-sets result, over 2.5 wins if it goes to a deciding set.
If a match cannot be finished, a trader can void it and your stake is returned.
Three surfaces, three kinds of tennis
Betting is entertainment, not income. Set a limit before you start, and take a break when you need one: limits and breaks.