U.S. Open champion Sam Stosur lost in the opening round of the Unicef Open on Monday, falling 7-6 (7), 6-3 to Kirsten Flipkens of Belgium.

Flipkens served nine aces while the top-seeded Stosur double-faulted seven times at the grass-court warmup for Wimbledon.

On Sunday, Kim Clijsters returned from a three-month hip injury to beat fifth-seeded Romina Oprandi of Switzerland 6-7 (5), 6-2, 6-3.

On the men's side, third-seeded Jurgen Melzer of Austria will play Igor Kunitsyn of Russia later Monday.