San Diego, March 3 (IANS): Marta Kostyuk of Ukraine stunned top seed American Jessica Pegula 7-6(4), 6-1 in the semifinals of the San Diego Open.

The 21-year-old Ukrainian overcame a 5-1 deficit in the first set to surge past the World No.5 in 1 hour and 35 minutes and will face Katie Boulter for the title on Sunday.

Pegula led the head-to-head 2-0 and looked primed to maintain her perfect record over Kostyuk. Down 5-1, Kostyuk increased her aggression and turned the tables quickly.

Kostyuk won four consecutive games to level the set at 5-5 and then wiped out a break point to hold to 6-5. Pegula held serve to force the tiebreak, but Kostyuk's momentum carried her through to seal the opening set.

Having settled on her winning game plan, Kostyuk continued her domination in the second set. She finished the match with 26 winners to 23 unforced errors. Pegula struck 16 winners to 19 unforced errors and was broken five times, WTA reports.

World No.49 Boulter, Kostyuk's final opponent, advanced to her first hard-court final by defeating No.3 seed Emma Navarro 6-3, 6-1.

The victory is Boulter's second win over a Top 3 seed this week, having defeated No.2 seed Beatriz Haddad Maia in the first round.

Sunday's final will be the first career meeting between Kostyuk and Boulter. Both players are bidding to capture their second career titles. Like Boulter, Kostyuk broke through to win her first title last year, winning in Austin.