Samantha Stosur has been sent out in the second round of the Cincinnati Masters, suffering another loss to world No.1 Serena Williams.

The Australian, who suffered a 6-0 6-2 defeat to Williams in less than an hour Montreal last week, proved a tougher opponent on Wednesday as she pushed the top seed to 7-6 (7) 7-6 (7).

Wimbledon champion Petra Kvitova crashed out in another setback to her US Open preparations.

Ukrainian teenager Elena Svitolina toppled the third-seeded Czech 6-2 7-6 (7-2).

Kvitova’s exit in her first match after enjoying a first-round bye came a week after a third-round exit on the hardcourts of Montreal, and the world No.4 admitted she couldn’t find her way against the Ukrainian, who is playing her first tournament since defending her title at Baku last month.

“I can still find moments when I really don’t know what’s happening out there,” Kvitova admitted.

“Even if I’m older and have more experience, I can still have days like today.”