LONDON—Stanislas Wawrinka explains why he complained to the chair umpire in the second set of his 7-6, 7-6 loss to Rafael Nadal Wednesday at the ATP World Tour Finals.

“It's nothing personal against Rafa or against [Nadal uncle and coach] Toni," Wawrinka said. "We all know, players and umpire, that Toni is always trying to help Rafa. That's normal. That's part of the game. But when it's too much, it's too much.

"Today I didn't agree with the umpire that he didn't tell him something or he didn't give him second warning just because it was Rafa. We all see. I was there. Before every point, he was trying to coach him. That just what happened. Again, it's nothing against Rafa or Toni. That's in the rules. Normally the umpire should have done something.”