Ash Barty let slip a chance to take both the first and second set to fall to No.14 seed Sofia Kenin in the Australian Open semi-final in sweltering heat.
Barty was ahead in the first set tiebreak but a string of errors let the energetic American take the first set in temperatures pushing 39C.
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The World No.1 steamed back in the second set and was serving for the match at 5-4 after some breathtaking tennis that saw Barty mix her slice in with devastating forehands.
But the resolute American broke a nervous Barty and then doubled down to break Barty again in the penultimate game to steal the match 7-6, 7-5.
The 21-year-old will play either fourth-seeded Wimbledon champion Simona Halep or resurgent Spaniard Garbine Muguruza in Saturday's final.
It's over! Kenin has taken down Ash Barty, and it's the American into the final! She is in complete disbelief!— Wide World of Sports (@wwos) January 30, 2020
Stream: https://t.co/VLoDqoHZ5m#9WWOS #AusOpen pic.twitter.com/WredQg39ye
That's gonna hurt for Ash, but what a match Kenin played. She is very smart.— José Morgado (@josemorgado) January 30, 2020
Ash Barty was so off her usual game this match. So uncharacteristic. #AusOpen— Danny R (@currentdanny) January 30, 2020
Barty to rue missed chances
While Kenin gets the chance to claim her maiden grand slam crown, Barty will rue being unable to convert either of two set points from 6-4 up in the first-set tiebreaker or another two serving at 5-4 in the second set.
A hot favourite to become the first Australian woman since Wendy Turnbull 40 years ago to make an Open final, the world No.1's defeat left the packed crowd on Rod Laver Arena shellshocked.
Even Kenin, who lost to Barty in the French Open fourth round last year, was pinching herself in the aftermath.
"Honestly, I'm so speechless. I honestly can't believe this. I've dreamed about this moment since I was five years old," said the Moscow-born 14th seed.
"I'm so speechless, thankful. I have worked so hard to be here. Thanks to my dad and everyone back home."
Barty dominated much of the opening set, conceding only four points in her first five service games.
But she paid the price for not cashing in on three break-point chances in the sixth game, as Kenin snatched the set in the breaker.
A winner of a tour-best 13 matches from a set down last year, Barty looked to be mounting a signature comeback when she charged to a 3-1 lead in the second set.
But, in temperatures nudging towards 40 degrees in Melbourne, the top seed crumbled trying to level the match up as Kenin reeled off four straight games from 3-5 down to seal victory after one hour and 45 minutes.
"I knew I needed to find a way to win. Thank you so much," Kenin told the crowd.
"This wasn't an easy one. I'm so grateful and thank you for this moment."