LONDON (AP) — Stanislas Wawrinka finds it hard enough playing against his nemesis Rafael Nadal — without the Spaniard receiving extra help from his coach.

Once again, the Swiss player failed to win a set against Nadal, losing 7-6 (5), 7-6 (6) to his top-ranked opponent at the ATP World Tour Finals on Wednesday.

It was his 12th loss in as many matches against the Spaniard.

Wawrinka, who can still reach the semifinals, regretted the fact that Nadal wasn't penalized for coaching by his uncle Toni. Nadal did receive a warning for slow play.

Wawrinka says "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."

Wawrinka adds "Before every point, he (Toni) was trying to coach him."