Two-time major champion Svetlana Kuznetsova double-faulted 10 times and was upset by 45th-ranked Dominika Cibulkova of Slovakia 7-5, 7-6 (4) in the fourth round of the U.S. Open on Monday.
Cibulkova reached her first quarterfinal at Flushing Meadows despite being treated by a trainer for a left leg problem between sets. She was the lowest-ranked of the 16 women in the fourth round.
Her best Grand Slam result was reaching the semifinals at last year's French Open. The 11th-seeded Kuznetsova won the 2009 title at Roland Garros, as well as the 2004 U.S. Open championship.
Cibulkova is 2-8 against top-15 players this season, and both victories came against Kuznetsova.