Top-seeded Agnieszka Radwanska slumped to a 7-6 (2), 6-2 defeat by American qualifier Jamie Hampton in the first round at Eastbourne on Tuesday.
Radwanska, who won the title in 2008, struggled throughout the match at the Wimbledon warm-up tournament.
Although the Polish player leveled at 5-5 when Hampton served for the first set at 5-4 and then held a set point at 6-5, she was overwhelmed in the tiebreaker and failed to make much impact in the second set.
"I think playing her in the first match on the grass I think it was tough, and I think, you know, the fact that she played three matches before helped her a little bit," Radwanska said. "I think I was a little bit scared to move. It wasn't slippery. Maybe a little bit in the back, but, yeah, scared. You have to be careful."
Hampton was cautious when Radwanska fought back late in the first set, but was determined not to fold.
"I was a little rattled, yeah," Hampton said. "Last time we played something similar happened, so I was like, I'm not going to crack. I'm going to hold on and get to a tiebreak."
Second-seeded Li Na of China advanced to the second round, beating Alize Cornet of France 6-2, 6-4 and will next play 2011 champion Marion Bartoli of France.
There were also victories for 2012 runner-up Angelique Kerber of Germany, who defeated Romania's Sorana Cirstea 6-4, 6-4 and fourth-seeded Czech Petra Kvitova, who dismissed Romanian Monica Niculescu 6-4, 6-1. Fifth-seeded Caroline Wozniacki of Denmark advanced after leading 6-2, 2-2 when defending champion Tamira Paszek of Austria retired with a left thigh injury.
Wozniacki faced a potentially difficult match against an opponent who not only won the Eastbourne title last year but then went on to defeat her in the first round at Wimbledon.
She is also emerging from a period during which she won just one of her last six clay court matches.
"I'm feeling great," Wozniacki said. "I love the grass. It's so much fun to play on. The clay season is over, which I'm happy about. I think I played a really good first set. I felt really good and comfortable out there."
In the men's event, fifth seeded South African Kevin Anderson was upset 7-6 (4) 6-2 by Julien Benneteau of France, and Australia's Bernard Tomic defeated Britain's James Ward 6-3, 6-4.