Caroline Wozniacki was upset by Petra Cetkovska in the second round of the 2015 U.S. Open on Thursday night at Arthur Ashe Stadium.
Cetkovska earned a 6-4, 5-7, 7-6(1) over Wozniacki, who was the runner-up at last year's Open.
Wozniacki, the tournament's No. 4 seed, got off to a slow start, losing the first set and dropping the first three games of the second. But Wozniacki rallied to force a third set and appeared to have Cetkovska on the ropes.
Down 6-5 and serving, Cetkovska saved four match points to force a tiebreaker, which the Czech Republic native won 7-1.
Cetkovska was also responsible for Wozniacki's second-round exit at Wimbledon in 2013.
Cetkovska will play No. 26 Flavia Pennetta in the third round.
- Mike Fiammetta and Stanley Kay