After a day of semifinal upsets at the Cymbiotika San Diego Open, Marta Kostyuk will face Katie Boulter for the title on Sunday.

No.34 Kostyuk notched the first Top 5 win of her career after mounting a comeback to defeat top-seeded Jessica Pegula 7-6(4), 6-1 in the semifinals. 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. 

"I don't even know how I turned this around," Kostyuk said on court. The win came 24 hours after the Ukrainian came from a set down to defeat Anastasia Pavlyuchenkova in the quarterfinals.  

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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. 

"I think I was just rushing a little bit in the beginning, just picking the wrong balls to accelerate or to come in on," Kostyuk said.

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. 

"Top 5 wins are much more difficult than Top 10 because girls in the Top 5 are usually much more consistent," Kostyuk said. "This one is extra special and important. I also lost twice to Jess before. So it definitely feels good."

No.49 Boulter 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. 

Ranked No.49 at the start of the week, Boulter will look to win her first WTA 500 title on Sunday. The 27-year-old from Leicester won her first Hologic WTA Tour title last year on the grass at Nottingham.  

Facing No.26 Navarro for the first time, Boulter dominated the surging American from the baseline to seal the win after 1 hour and 14 minutes. Boulter struck 24 winners and held Navarro to just eight. The victory improved Boulter to an impressive 6-1 against seeded opponents this season. 

"Today was one of those matches where everything just clicks," Boulter said. "I came on the court and started the first few games and I could feel the power that she had and the way she works and maneuvers the ball. I knew I had to come up with something better."

Sunday's final will be the first career meeting between Kostyuk and Boulter. Both players are enjoying strong starts to their seasons. In January, Boulter signaled her intent with her first Top 5 win, beating Pegula at the United Cup. Kostyuk notched a milestone of her own, advancing to her first major quarterfinal at the Australian Open.

Both are bidding to capture their second career titles. Like Boulter, Kostyuk broke through to win her first title last year, winning in Austin. 

"I was following this week how she's been playing and she's been playing really, really good, very aggressive, very flat," Kostyuk said. "So again, I have to stay low tomorrow. It will be another great fight."