Set spreads in tennis are placed at -1.5 or +1.5, much like the run line in football.
Game Example: Nadal vs Alcaraz : Nadal -1.5 Sets
In the following example, if you bet on Nadal at -1.5 sets and he wins in straight sets 2-0, you win the bet. If Alcaraz wins a set, you will lose.
Game Example: Svitolina vs Muguruza : Muguruza +1.5 Sets
In the following example, if you bet on Muguruza at +1.5 sets and she wins 1 set, you win the bet. If Svitolina wins in straight sets 2-0, you will lose.