Three-time Grand Slam champion Stan Wawrinka, struggling in his comeback from left knee surgery, was ousted by US qualifier Donald Young at the ATP and WTA Washington Open on Tuesday.

Young, ranked 234th, outlasted the 33-year-old Swiss 6-4, 6-7 (5/7), 7-6 (7/3) and advanced to a second-round match with Japanese seventh seed Kei Nishikori.

"It was a tough fight, a lot of nerves because he can hit any shot," Young said. "Stan at the top is totally different than when he's working his way back. I'm sure he'll be back. I won't want to play him when he's back."

Wawrinka, ranked 198th, fell to 6-11 this year after missing most of nine months before returning in May.

"It's painful to lose in the first round here, especially 7-6 in the third," Wawrinka said. "I was missing a lot for sure. It's tough when you haven't played a lot of matches. Tough loss for sure."

On the women's side, reigning US Open champion Sloane Stephens opened by beating fellow American Bethanie Mattek-Sands 7-5, 6-4.

Third-ranked Stephens, who captured her first Grand Slam title last year in New York, rallied from 5-2 down in the first set to book a second-round match against Germany's Andrea Petkovic.

"I'm super excited about the hardcourt season again," Stephens said. "It was good I was able to fight back and win."

Wawrinka, who began his career in 2002, won his first Grand Slam title at the 2014 Australian Open and added the 2015 French Open and 2016 US Open titles. He reached the Wimbledon quarter-finals in 2014 and 2015 but hasn't gotten past the second round since.

Hard-learned patience over 12 years before his Slam breakthrough is paying dividends as Wawrinka tries to recapture top form.

"There are different steps. Right now is to get back a little confidence and get rid of all the doubt you have in your game," Wawrinka said.

"I feel like I'm really close, but sometimes really far. I'm sure I'll get back where I want to be. It's a long process and you have to accept that. I know I'll get back. I just don't know how long it will take."

It doesn't help that Roger Federer and Rafael Nadal returned from injuries and quckly found top form. After several knee injuries, Nadal has won three of the past six Grand Slam titles. Federer has taken three of the past seven Slam titles after 2016 knee surgery.

"You can't compare an injury," Wawrinka said. "Some come back quickly. Some it takes more time.

"You need to accept it's going to be painful. It took a lot of years to win Grand Slams and if it takes me a few months to get my game back in top shape then that's all right."

- No. 2 Wozniacki withdraws -

Stephens, who won her first WTA title at Washington in 2015, fired her sixth ace on her first match-point chance to advance after one hour and 53 minutes.

"I knew I was in for a real test from the very beginning," Stephens said. "I knew I had to be patient, play my game and wait for my opportunities."

Stephens became the top-rated player in the draw when world number two Caroline Wozniacki, the reigning Australian Open champion, withdrew due to a right leg injury.

"It's just something that has been nagging for a little while," Wozniacki said. "Hopefully it will be fine."

Stephens' top-rated remaining rival is Japan's 17th-ranked Naomi Osaka, the reigning Indian Wells champion who won her Washington debut 6-2, 7-6 (7/4) over American Bernarda Pera.

"Confidence-wise, winning Indian Wells was good for me," Osaka said. "It has helped me a lot."

Top-seeded defending champion Alexander Zverev of Germany was scheduled to open later in a second-round match against Tunisia's Malek Jaziri. Third-ranked Zverev beat Jaziri in last year's third round at Washington.

Zverev has never played brother Mischa, the 15th seed, in an ATP event but they could meet in the third round, with Mischa to face US wildcard Tim Smyczek on Wednesday.

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