Belinda Bencic of Switzerland beat Daria Kasatkina of Russia 6-4, 6-3 in a battle of 18-year-olds to book a St. Petersburg final against Roberta Vinci of Italy, who earlier in the day downed Serbia's Ana Ivanovic in straight sets.

Bencic came under pressure from Kasatkina but secured a win that ensures she will debut in the WTA top 10 Monday regardless of the result of Sunday's final.

"This is a huge moment for me. To be in the top 10 is always like a dream for every small kid," Bencic, currently ranked 11th in the world, said in televised comments.

She said she enjoyed her first professional match against Kasatkina, a longtime friend from their juniors days.

"Dasha is a very good friend and I'm happy we can still be friends off the court," Bencic said, using Kasatkina's nickname.

The Russian had played a tough three-set quarterfinal against Dominika Cibulkova the day before, as well as a doubles match that lasted until 1 a.m., and needed treatment on her back and stomach muscles in the second set. However, she fought back, repeatedly coming close to breaking Bencic before the Swiss closed out the match on Kasatkina's serve.

The final pits the top two seeds, with Bencic in her first tournament as the first seed.

Earlier in the other semifinal, Vinci topped Ivanovic 7-5, 6-4. Vinci, ranked 16th in the world, was 3-0 down in the first set before recovering to win both sets, recording just 12 unforced errors to 29 for Ivanovic.

Vinci said it was "an incredible match" and that she was "so happy" to make the final of the inaugural St. Petersburg tournament.

Vinci is 9-5 in career tour singles finals but has lost four since her last win in Palermo in 2013, including last year's U.S. Open final. Bencic is 2-3 in WTA finals.