Petra Kvitova started her bid for a fourth Connecticut Open crown with a straight-sets win over Agnieszka Radwanska, while world number one Simona Halep withdrew from the WTA tournament.

A three-time winner in New Haven, Czech star Kvitova was too good for former world number two and 2016 champion Radwanska 6-1 7-6 (7-3) on Monday.

Seeded third, two-time Wimbledon champion Kvitova rallied from 3-1 down in the second set of her opening match to earn a meeting with qualifier Zarina Diyas.

"I'm glad that I made it in the two [sets] for sure," Kvitova said. "I did not want to be on the court much longer to play against Aga. She's a great mover on the court and gets to a lot of balls so it could have been a long match. I had to fight in the second set when I was down 3-1 but just waited my chance to break her.

"I think once she played longer she played better and better. My serve went pretty OK. I'm glad that I found my rhythm in the end of the second set. I had some easier games on serves and that was saving my energy as well."

Earlier in the day, French Open champion Halep pulled out due to an Achilles injury following her loss to Kiki Bertens in Sunday's Cincinnati Masters final.

The Connecticut Open lost another big name on Monday after fourth seed Karolina Pliskova suffered a 6-1 6-3 defeat against Ekaterina Makarova.

Johanna Konta accounted for Laura Siegemund 6-2 7-5, Aryna Sabalenka topped Samantha Stosur 6-3 6-2, Camila Giorgi eased past Ana Bogdan 6-3 6-2, Danielle Collins lost 6-0 6-3 to Dayana Yastremska, Aliaksandra Sasnovich outlasted Kristina Mladenovic 7-6 (8-6) 6-7 (3-7) 6-2, while Monica Puig beat Timea Bacsinszky 7-5 6-1.