Aug 20 (Reuters) - World number three Jessica Pegula survived a stirring fightback from Amanda Anisimova to win 6-4 2-6 7-6(4) and reach the Cincinnati Open semi-finals on Thursday, while Iga Swiatek also advanced when Elena Rybakina retired injured.
World number two Rybakina retired in the first set with an ankle problem in a setback to her preparations for the U.S. Open Grand Slam, which begins on August 30.
In the men's draw, seventh seed Flavio Cobolli recovered from a sluggish start to beat home
favourite Tommy Paul 2-6 6-3 6-4 and reach his first ATP Masters 1000 semi-final.
His opponent will be Arthur Fils after the Frenchman swept past Argentina's Thiago Agustin Tirante 6-3 6-2.
Pegula took the opening set in 34 minutes but ninth seed Anisimova hit back hard to claim the second, breaking early for a 3-0 lead before Pegula took a medical timeout.
American Pegula squandered a 5-3 advantage in the decider but prevailed in the tiebreak to set up a meeting with Swiatek, who extended her winning run to 10 matches when world number two Rybakina retired while trailing 4-1 in the opening set.
Swiatek raced into a 3-0 lead before Rybakina got on the board, but the Kazakh was unable to halt Swiatek's momentum and called for a medical timeout before ending the match.
The pair had also met in last week's Toronto final, where Swiatek beat Rybakina in straight sets to claim the Canadian Open title.
"I hope Elena will be fine, and obviously the next tournament is the priority," Swiatek said. "I know this one is important as well, but the U.S. Open is the U.S. Open, and I hope she'll be ready for it. I thought we were playing a good game, but health is a priority."
Cobolli, who became only the second Italian to reach the men's last four in Cincinnati after Jannik Sinner, asked for a tub of M&M's while leading 5-2 in the second set, later crediting the snack with giving him fresh energy.
"I felt a little bit tired, and I needed some energy, some sugar, and I love M&M's," he said. "Chocolate with colours makes me happy."
Cobolli broke to move 5-4 ahead in the deciding set before serving out the match in one hour and 57 minutes.
Frenchman Fils broke serve to move 5-3 ahead before wrapping up the opening set and broke twice more in the second, racing to a 5-1 lead and closing out the match in 64 minutes.
(Reporting by Nicole Fernandes in Toronto and Angelica Medina in Mexico City; Editing by Ken Ferris and Peter Rutherford)
