World number one Caroline Wozniacki lost her opening match for the second tournament in a row, beaten 6-4 7-5 by American teenager Christina McHale at the Western and Southern Open in Cincinnati.
Wozniacki, who also fell at the first hurdle in Toronto last week on her return from a month out with a shoulder injury, let slip a 5-1 lead in the second set as McHale reeled off six games in a row to seal victory.
The defeat was the first time Wozniacki has lost to an opponent outside the top 75 since 2009 but the Dane remains confident of turning things around before the US Open, which starts on August 29.
"Such things happen during a season, to have highs and lows," said Wozniacki, who plans to play in New Haven next week.
"I am going to come back stronger and win matches very soon.
"I cannot remember the last time that I lost my first match on successive occasions. But such things happen.
"Christina served well on the important points and I committed unforced errors that I shouldn't have.
"I made 37 when I usually commit seven or eight in a match. However, life goes on.
"I would have liked to have more matches under my belt before the US Open but I am going to train even harder."
Also on Wednesday, Croatia's Petra Martic claimed the scalp of 17th seed Yanina Wickmayer.
But seventh seed Francesca Schiavone, 14th seed Svetlana Kuznetsova and 16th seed Shuai Peng all advanced to the third round.