US Open winner Sloane Stephens suffered a shock defeat at the hands of China's Wang Qiang in the first round of the Wuhan Open on Monday, hours after fellow Flushing Meadows finalist Madison Keys tumbled out.

Stephens, the 14th seed, lost 6-2, 6-2 in her first WTA event since her surprise victory against Keys in New York.

Despite a strong fight by Stephens in the second set, Wang -- spurred on by the home crowd in the 15,000-capacity stadium -- proved too strong for the 24-year-old American. 

"I didn't play a great match. But my opponent played very well," Stephens told a post-match press conference.

"I mean, it's difficult. It's the end of the year. Even though I played five tournaments, I feel like I played 25."

Stephens, who ranks number 17 in the world, added she had been taking it easy since her US Open win.

"I went to see my grandparents. So no partying for me. I don't party. I don't drink. I don't do any of that stuff," she added.  

Keys also crashed out at the first hurdle in the hot and steamy Chinese city, losing to US qualifier Varvara Lepchenko after struggling to overcome a longstanding wrist injury.

The 22-year-old American 10th seed fell 6-2, 7-6 in what was also her first WTA tournament since the Flushing Meadows final.

Keys, the world number 12, called a medical timeout early in the second set and looked close to tears as she examined her left wrist, which has twice needed surgery in the past. 

Keys had chances to level the match in the second set but Lepchenko saved two set points to hold at 5-5, before taking the tiebreak 7-4.

"The strength (of my wrist) has been the biggest issue... I've been dealing with it over the summer," she told journalists. 

"But we kind of all knew it (would be like this) until we had a solid chunk of time to strengthen it."

Elsewhere, Agnieszka Radwanska, the ninth seed, beat Magdalena Rybarikova 6-4, 7-5.

Lepchenko, ranked 79 in the world, will play either Kiki Bertens of the Netherlands or Romanian Monica Niculescu in the second round.

Wang, ranked 57, will play Romanian Sorana Cirstea.