Caroline Wozniacki was knocked out of the St. Petersburg Ladies Trophy by Dominika Cibulkova 6-4, 7-5 after a heated dispute with the umpire over a mistaken call.
Wozniacki was 5-3 down in the second set when a Cibulkova shot was wrongly called out.
Umpire Emmanuel Joseph overruled the call and awarded the point to Cibulkova instead of replaying the point even though Wozniacki had clearly returned the ball in court Thursday.
That prompted a lengthy dispute between the Danish player and the umpire. Despite not getting the point, the controversy seemed to fire up Wozniacki, who saved four match points and broke Cibulkova's serve before Cibulkova eventually closed out the match in straight sets.
"I was aggressive. Even if I did some mistakes I was really confident. It was the only way I could beat her," Cibulkova, of Slovakia, said in televised comments. "I'm happy with the way I stayed really tough mentally."
Cibulkova, the Australian Open losing finalist in 2014, next plays Darya Kasatkina in the quarterfinal after the 18-year-old Russian beat Laura Siegemund of Germany 6-3, 6-4.
Top-seeded Belinda Bencic overcame serving issues earlier in the day to defeat Annika Beck of Germany 7-6 (3), 6-3.
Bencic was broken four times in a chaotic first set in which she recorded six double faults, including one on break point that allowed Beck to serve for the set at 6-5. However, the Swiss recovered to break Beck and dominate both the resulting tiebreak and second set.
"I had some trouble in the start and I think the court was very slow," the 18-year-old Bencic said in televised comments. "My timing was a little bit off."
Bencic, who had a first-round bye, next plays a quarterfinal against either fifth-seeded Anastasia Pavlyuchenkova of Russia or Carina Witthoeft of Germany.
Earlier, Hungary's Timea Babos defeated ninth-seeded Monica Niculescu of Romania 4-6, 6-1, 6-3 in her second-round match.