ST. PETERSBURG, Russia - Top-seeded Belinda Bencic won a late-night battle with Anastasia Pavlyuchenkova to take the final spot in the semifinals at the St. Petersburg Ladies Trophy.

Over the course of a two-and-a-half-hour match which concluded after midnight, the Swiss world No. 11 ground down her fifth-seeded Russian opponent to win 6-7 (4), 6-4, 6-2.

While Bencic's serving was patchy, she made up for it with tenacity in rallies off her second serve, and was a constant threat in Pavlyuchenkova's service games, forcing 18 break points, of which 11 were converted.

Bencic's win sets up a semifinal against a fellow 18-year-old in Russia's Daria Kasatkina, who overcame the tenacious Dominika Cibulkova of Slovakia 4-6, 7-5, 6-3.

The other semifinal pits second-seeded Roberta Vinci of Italy against Ana Ivanovic, the Serbian fourth seed.

Vinci survived a scare from Timea Babos of Hungary to reach the semifinals. The Italian broke Babos when the latter was serving for the match in the deciding set, and bounced back from 4-2 down in the tiebreak to win 7-6 (3), 4-6, 7-6 (4).

Vinci struggled throughout the first and second sets, landing fewer than half of her first serves in each.

"She served so good, but today I played so-so. Especially at the beginning, I was so tight, so nervous," Vinci said in televised comments. "I stayed focused and I won a great match."

Vinci meets Ivanovic after the Serb beat Ukrainian qualifier Kateryna Kozlova 6-1, 7-5. Ivanovic limited Kozlova to just 10 points in the whole of a rapid first set but faced a tougher task when Kozlova broke her serve twice in the second.

"She is a great young player, and in the second set she started to play more powerful," Ivanovic said. "I tried to step up a bit, be more aggressive, hold my court position, and it worked well."