Australian Open champion Aryna Sabalenka lost for the first time in 14 matches, and Coco Gauff set up a semi-final clash with Iga Swiatek at the Dubai Tennis Championships on Thursday.

Sabalenka fought back from a set down to end the title defence of Jelena Ostapenko a day earlier, but she came up short against Barbora Krejcikova as her Czech opponent overcame a tough first set to run out a 0-6 7-6 (7-2) 6-1 victor.

Sabalenka, playing in her first event since claiming a maiden grand slam title in Melbourne, romped into the lead by breaking Krejcikova's serve three times to go a set ahead.

But the world number two was pegged back in the second as Krejcikova battled to force a tie-break, which she dominated to level the match.

Krejcikova then completed the shock victory in style, breaking the second seed twice in the final set before clinching the win on her third match point to end Sabalenka's run of 13 straight triumphs.

Karolina Muchova's withdrawal due to an abdominal injury means it is Jessica Pegula who will face Krejcikova in the semi-finals on Friday.

After Swiatek reached the final four by virtue of Karolina Pliskova pulling out because of an illness, Gauff set up a sixth meeting with the world number one by beating Madison Keys 6-2 7-5.

In those five previous clashes between the pair, Gauff is yet to win a set.

But after cruising past Keys, doing so by converting four of five break points and winning 90 per cent of her service games, Gauff is confident of improving her dismal record against Swiatek on Friday.

"All five times, I did something wrong," Gauff said. "To be honest, she's playing great tennis and there's a reason she's world number one. 

"Tomorrow I have no pressure. I just have to play my game. I definitely think I've gotten better since the last time I played her.

"Ranking is just a number at the end of the day. You just have to step on the court believing you can win, and that's what I'm going to do tomorrow."