Defending champion Novak Djokovic advanced to the quarterfinals of the U.S. Open when Stanislas Wawrinka retired while trailing in the third set.

Djokovic was leading 6-4, 6-1, 3-1 on Wednesday when Wawrinka ended the match because of illness. Wawrinka said he had been sick for three days and didn't want to risk worsening his condition before the Swiss Davis Cup match next week.

The second-seeded Serb was up 2-0 in the first set Tuesday when play was suspended because of rain. He spent a total of just 93 minutes on court to reach his 14th consecutive Grand Slam quarterfinal.

Wawrinka, seeded 18th, has lost 19 straight matches against top-three opponents.