CINCINNATI, OH, USA - Coupe Rogers quarterfinalist Kiki Bertens won the first set of her second round clash with No.2 seed Caroline Wozniacki when the reigning Australian Open champion was forced to retire with a left knee injury, trailing, 6-4, at the Western & Southern Open.

READ: Bertens - 'I'm doing things my way this year'

"I'm sorry that she had to retire," Bertens said. "I hope she'll be better soon and ready for New York. From my side, it's a little bit tough to play at night. I'd rather play during the day when it's warm, but I was just trying to play aggressive."

Bertens has been the in-form player for much of the last six months, winning the Volvo Car Open, reaching her first Premier Mandatory final at the Mutua Madrid Open and her first Wimbledon quarterfinal just three weeks ago.

"I just keep working every day, and that's working well this year. I'll keep doing that."

Wozniacki, by contrast, has struggled with injury since Wimbledon, withdrawing from the Citi Open and falling in the second round in Montréal to Aryna Sabalenka.

"It's never nice to win when someone has to retire. I felt she was struggling a bit with her serve; she wasn't serving as well as she usually does. She seemed to struggle with her movement as well."

Delayed by rain for over three hours, Wozniacki called the trainer after losing serve for 5-4 in the first set, and retired shortly after Bertens captured the opening set in 62 minutes.

"I just took a nap, played some board games, and relaxed. We just hung out and tried to be ready," Bertens said of the rain delay.

"I was trying to keep focusing on what I had to do: play aggressive, go for my serve and shots. That was it."

Clearly out of sorts with a wrapped right shoulder in addition to the knee issue, the former World No.1 struck 12 unforced errors and no winners, while Bertens stepped into the court for 9 winners to 14 unforced errors.

"I'm really happy with my form. We practiced the whole year to play more aggressively for hardcourts and grass. It's really working out."

Up next for Bertens is Estonia's Anett Kontaveit, who dispatched Maria Sakkari in straight sets on Wednesday.

Bertens and Kontaveit have split their two previous meetings, with the Dutch star winning their last encounter at the Ladies Championship Gstaad in 2017, while Kontaveit won their only hardcourt match at the Connecticut Open two years ago.

"She's a great player. I've played her in the past and I know her pretty well. It's going to be a good match; she's played well here. Hopefully we can play a good match tomorrow."