BEIJING (AP) A teary-eyed Eugenie Bouchard retired from her opening match against Andrea Petkovic at the China Open due to dizziness on Monday, her first match since falling at the U.S. Open last month and suffering a concussion.

Bouchard slipped in the locker room after her third-round win against Dominika Cibulkova at the U.S. Open, falling backward and landing hard on the back of her head. She was forced to withdraw from the tournament, as well as subsequent tournaments in Tokyo and Wuhan, China.

The Canadian was trailing 6-2, 1-1 against Petkovic on Monday when she decided to call it quits.

''I thought I was physically ready, but unfortunately the symptoms of my concussion came back,'' Bouchard said in a statement. ''I hope to get well soon and look forward to returning next year.''

After Bouchard retired from the match, Petkovic came around the net to check on her.

''I just asked her what happened, and she told me that she felt very dizzy,'' the German said. ''Then I asked if it was the same, if it still was the concussion. She said, `Yeah, it tends to come back when she gets physically very active.'''

It's been a difficult year for Bouchard following her breakout season in 2014 when she reached the Wimbledon final and the semis at the Australian and French Open. Her record in 2015, meanwhile, is just 12-17 and her ranking has fallen all the way from No. 5 to No. 38.

The Canadian's retirement came on the heels of four other withdrawals in the women's draw on Sunday, including top-seeded Simona Halep, who retired from her match against Spain's Lara Arruabarrena with a recurring Achilles injury.

Defending champion Maria Sharapova and world No. 1 Serena Williams also withdrew before the tournament began, citing injuries.

In women's second-round matches on Monday, U.S. Open runner-up Roberta Vinci of Italy defeated Alison Van Uytvanck of Belgium 6-2, 6-2, Madison Keys of the United States rallied to beat Kateryna Bondarenko of Ukraine 3-6, 6-1, 6-0, and seventh-seeded Carla Suarez Navarro ousted Puerto Rico's Monica Puig 7-6 (1), 6-2.

In a first-round match, Switzerland's Belinda Bencic outlasted Madison Brengle of the United States 6-7 (4), 6-4, 6-3.

In the men's draw, eighth-seeded Jo-Wilfried Tsonga of France was upset by Austria's Andreas Haider-Maurer 7-6 (4), 6-2.

Tsonga said afterward that he, too, felt dizzy during the match and he was asked if the heavy pollution in Beijing on Monday had affected his play. According to the U.S. Embassy's air quality measurements, the PM 2.5 reading hit 366 in the evening, which is considered ''hazardous'' on the index.

''I don't know. You know, nothing in me can calculate if it's enough oxygen for me or not,'' he said. ''Today I had an opponent, he was just better than me today.''

In other first-round matches, Serbia's Viktor Troicki upset fifth-seeded Milos Raonic of Canada 6-4, 6-4, seventh-seeded David Goffin of Belgium defeated Italy's Andreas Seppi 6-2, 6-3, and Chinese wild card Zhang Ze, ranked No. 219 in the world, beat 70th-ranked Denis Istomin of Uzbekistan 6-3, 1-6, 6-3.