The Russian, ranked No 24 in the world, saves three match points before beating the No 2 seed 3-6, 7-6, 6-1 to reach Saturday's final.
Daria Kasatkina fights back to defeat Garbine Muguruza and reach WTA Dubai Duty Free Tennis Championships final
World No 24 Daria Kasatkina upset reigning Wimbledon champion Garbine Muguruza on Friday to book a spot in Saturday's final of the WTA Dubai Duty Free Tennis Championships.
The Russian, at age 20 the youngest Dubai semi-finalist since 2010, fought back from losing the first set at the Aviation Club and having to save three match points in the second set to take a pulsating match 3-6, 7-6, 6-1.
Having battled past the world No 3, Kasatkina will now face either defending champion Elina Svitolina or former world No 1 Angelique Kerber in the final. They play later on Friday night.
Asked how she came through the match, Kasatkina, who has a solitary WTA Tour title to date, said: “Who knows? When you're winning from match points down, you don't know why.
“I was just trying to fight for every ball because Garbine, she's playing unbelievable. She's a very tough opponent. She's hitting so hard, playing so fast. I was just trying to do whatever I could.
“[Tomorrow] is a final already, so I have to put everything what’s left in my body into this match. I hope I will.”
Following a relatively straightforward opening set for Muguruza - the Spaniard broke in the sixth game to go 4-2 up - Kasatkina survived a couple of breaks and then three match points in a thrilling tie-break to take the second.
She broke Muguruza in the opening game of the deciding set, and then again in the fifth to race to a 4-1 lead. Eventually, Kasatkina broke again to seal her place in a third career final in two hours and 33 minutes.
Afterwards she was in little doubt as to what had pleased her most.
“The fighting spirit, of course,” said Kasatkina, who has now saved match points in two rounds this. “Some things I was not able to do. Sometimes my forehand was not working, and the backhand I was pushing today. The fighting spirit made the difference, I think.
“All the work pays off. That's it. It's the golden rule, I guess.”
It represented a painful end to the week for Muguruza, with the two-time grand slam champion losing the Qatar Open final on Sunday. The No 2 seed in Dubai, she was unable to get past Kasatkina for a second time this year. The pair met in Brisbane last month, with Muguruza triumphing.
"She likes to play when she's under pressure," Muguruza said. "Her best tennis comes when she's about to lose, I feel. With the matches she normally plays, she's always in danger, and all of a sudden she kind of comes back.
"She has a good fitness to be able to run a lot. I think that's what helped her today. I was expecting a very difficult match. Like you said, the first one it went my side, this one her side. I'm looking forward for the third one."
Of her past two weeks' work, Muguruza said: "I think very positive because I managed to play a lot of matches, good ones, final and semi-final. It's a great two weeks. I'm happy to leave from Dubai with the feeling that I gave everything I had.
"I'm going to go as soon [to the United States] as possible. I want to prepare, rest a little bit. I feel like I played a lot of matches in the row, especially at night. A little bit of fatigue. I'm going to deserve be just laying down at some hotel, swimming pool."