CINCINNATI (AFP) — Iga Swiatek moved into the semifinals of the ATP-WTA Cincinnati Masters as Elena Rybakina was forced to retire with a heel injury less than half an hour into their clash Thursday.

Rybakina’s withdrawal raised concerns over the fitness of the world number two Kazakh just 10 days before the start of the US Open.

Trailing 4-1 in the opening set of the quarterfinal, Rybakina suddenly pulled up short at 40-all, heading to her bench and consulting with the trainer.

She went back out and played one more point before quitting.

“I was feeling anxious watching the whole situation,” said her opponent Swiatek. “I hope she will be fine. Health is always the priority.”

“This is an important tournament, but the US Open is the US Open. I hope she is ready for that,” she said, referring to the major which begins in New York on 30 August.

“It was a shame she was injured, we were having a good match.”

Had two-time Grand Slam winner Rybakina reached the Cincinnati final she would have risen to number one in the world for the first time in her career, following incumbent Aryna Sabalenka’s shock exit to 20-year-old Sara Bejlek.

Defending Cincinnati champion Swiatek said she had “gained a lot of confidence in the last weeks,” which have also included her victory over Rybakina in the final of this month’s Toronto event.

“I feel great, ready to play my game... I’m happy to be playing the tennis that I am.”

Poland’s Swiatek now plays a semi-final against Jessica Pegula, who is reportedly carrying a hip niggle after defeating Amanda Anisimova 6-2, 4-6, 7-6 (7/4).

Sixth-seeded Anisimova converted all five of her break points against Pegula, breaking her three times in the second set, before the world No. 3 rebounded to win.

Pegula is setting WTA records at age 32. She is the fourth player since 2009 to reach five or more Masters 1000 quarterfinals after turning 30, along with Li Na, Serena Williams and Svetlana Kuznetsova.

Pegula needed just 34 minutes to pocket the opening set.

But Anisimova struck back in the second, earning a double break for 4-0.

After Anisimova squared the match at one-set each, Pegula regrouped with an opening break in the deciding set.

They would each be broken twice as they went to the tiebreaker, where Pegula sealed the win on her first match point.

Pegula’s win was her sixth without a loss to Anisimova.

“I know I have a good record against her, but literally every single match is like it was today,” she said.

In the men’s draw, Flavio Cobolli advanced to the final four with a 2-6, 6-3, 6-4 defeat of Tommy Paul, gobbling down a handful of sweets for an unorthodox boost midway through the match.

The Roland Garros runner-up asked for the candy to be brought to the court and seemingly took energy from the pause before dispatching Paul, who had himself eliminated top seed Alexander Zverev 24 hours earlier.

“A sweet with chocolate makes me happy,” Cobolli said. “I’ve tried it in doubles so why not singles?”

Cobolli mounted the comeback after dropping the opening set.

With the match tied at a set apiece, Paul needed an extraordinary effort to hold serve for 4-3 in the final set, battling through five deuces and saving four break points.

But Cobolli ignored the showing and closed out the victory.

“It was a difficult day,” the winner said. “I’m proud of my effort, my game and my mental strength. It is all good practice for the US Open.”

Cobelli will play for a place in the final when he faces Arthur Fils, who defeated Thiago Tirante 6-3, 6-2 in just 64 minutes — the shortest men’s match of the event.

Fils, the eighth Frenchman to reach Cincinnati semi-finals in the Open Era, said he was impressed by the weight of his opponent’s big game.