When a tennis player plays a match, he exhausts himself and his physical shape bar decreases. The longer the match, the more tired he will be.
When he faces stressful events, like losing a set, or when a supposedly easy match lasts too much, the mental shape bar will empty itself. The opposite effect is also true: winning a set will refill the mental shape bar.
The ranking gap between 2 players can have a deep mental impact as well.
It's the same when your tennis player reaches for the first time a certain round / level of tournament.