Mensik secured the biggest win of his career when he rallied to defeat the world number two 7-6(3), 2-6, 6-3 under the Doha lights.
Playing behind a powerful serve, the Czech dug himself out of a hole in the third game of the opening set, saving four break points to hold serve. Mensik went on to clinch the tiebreaker.
While he dropped his first-serve percentage in the second set disaster, Mensik was back to his best in the third, winning 88 per cent of his first-serve points. The 20-year-old broke Sinner in the first and last games of the final set to clinch his first-ever win over a top-three-ranked player.
“I served pretty good, even if the conditions were starting to get slower during the night,” he said on court afterwards.
“It was a very tough mental battle. After the second set, when I dropped my energy a little bit, I’m really happy that I came back after the toilet break and from the beginning of the third set, I just started to serve well again. I’m just happy with the performance I showed today.”
The watershed win books Mensik a final four date with rising French star Arthur Fils.
Meanwhile, Alcaraz came back from a set down to defeat Karen Khachanov 6-7(1), 6-4, 6-3 to seal his semifinal spot at the hard-court ATP 500 event.
It was a thrilling display of baseline hitting, where Alcaraz racked up 41 winners and gave zero breakpoint opportunities to his opponent.
“It was a really close and tight match,” said the Australian Open champion.
“In the first set I had a few break points and I couldn’t make them. One set point and I couldn’t make it, so it was really tough.
“I think Karen was playing great, but I knew I’d had my chances. I just didn’t take them in the first set, so it was the time to keep going and try to keep fighting. Wait for my chances and try to take them in the second set. I’m just really proud about the way that I [fought].”
Alcaraz faces fifth seed Audrey Rublev in the semifinals on Friday night.
