Wimbledon champion Petra Kvitova crashed out of the second round of the WTA and ATP Masters hardcourt tournament on Wednesday in another setback to her US Open preparations.
Ukrainian teenager Elina 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 number four 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," admitted Kvitova. "Even if I'm older and have more experience, I can still have days like today."
Kvitova held a 3-0 lead in the second set as the Czech attempted a comeback effort. But Svitolina, ranked 39th in the world, nudged the scoreline back to a 5-4 lead only to be broken as she served for the victory.
The pair traded two more breaks before the tiebreaker, where Svitolina claimed victory on the first of four match points as Kvitova mis-hit a return wide.
"From the beginning I didn't feel very comfortable out there," Kvitova said. "And I was trying to really fight, I just wasn't able to do that today."
Rising Canadian star Eugenie Bouchard, runner-up to Kvitova at Wimbledon, was also in action Wednesday, taking on Russian veteran Svetlana Kuznetsova.
Top seed Serena Williams got through safely as she defeated Australian Samantha Stosur for the second time in as many weeks, 7-6 (9-7), 7-6 (9-7).
Fourth seed Agnieszka Radwanska beat Japan's Kurumi Nara 6-2, 6-2.
Stosur, who beat Williams in the 2011 US Open final, made the world number one fight for her win.
"She was up in both of the breakers, it was just a great match, to be honest," said the winner. "She served unbelievable, it was really intense."
German Angelique Kerber, the sixth seed, advanced against Ekaterina Makarova 6-4, 6-1 while Italian 13th seed Flavia Pennetta beat Taylor Townsend of the US 6-4, 6-3 to set up a clash with Williams.
Sabine Lisicki, the 2013 Wimbledon runner-up, beat Italy's Sara Errani, the 14th seed, 6-4, 2-6, 7-6 (7/2).