There are no point spreads in tennis because there is no meaningful way to measure how much a player wins or loses by. In tennis, all that really matters is who wins and loses, and that's where money line wagering comes into play.
Money line results are decided by an event's straight-up winner, without regard to any point spread, since there is no point spread.
For example, let's take the case of Serena Williams versus an overmatched opponent. A reasonable money line for this match would be listed as:
Serena Williams: -400
Opponent : +360
It would require Serena bettors to risk $400 to win $100; while a $100 bet on her opponent would win $360.
So every $400 bet on Serena nets a $100 profit if she wins (plus the return of the $400 risked). If her opponent pulls off the upset, $100 bet on the underdog would profit $360 (plus the return of the $100 risked).