Wawrinka advances as Djokovic quits, Federer rolls at US Open

Top-ranked defending champion Novak Djokovic retired from his US Open match Sunday after dropping the first two sets to Stan Wawrinka, sending the Swiss three-time Grand Slam champion into the quarter-finals.

Djokovic, who won four of the past five Slam crowns and 16 overall, was in pain from a nagging left shoulder injury while 34-year-old Wawrinka, who also beat Djokovic in the 2016 US Open final, showed himself a true title contender.

"It helped me big time knowing I have the game to beat him on that court," Wawrinka said.

"It's never the way you want to finish a match. I'm sorry for Novak. He's an amazing champion."
"It's frustrating. Very frustrating," Djokovic said. "Obviously not the first, not the last player to get injured and withdraw from one of the biggest events in sport."
"You just know (to quit) when you know, I guess, when you feel like you're not able to hit the shot anymore."
"It has been really tough since my surgery. It has took me two years to be back at that level. It feels really incredible."
"When I came to practice I was moving well, playing well. I was quite confident with the level I had but you never know when you are playing the number one player in the world."
"He's a great athlete playing super good tennis," Wawrinka said. "I'm sure it will be a great match."
- 'Ten times tougher' -
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"You catch a good day, the opponent doesn't, then things happen very quickly... I found my groove after a while and was able to roll really. Never looked back."
"All of a sudden every shot is 10 times tougher than usually. All of a sudden what you felt the day before during practice or the last matches, it feels completely different."