On Friday, Mensik will face Arthur Fils, who earlier defeated fellow Czech Jiri Lehecka, for a place in the final.
Mensik started the match fearlessly and aggressively, confidently holding his opening service game. However, he quickly found himself in trouble in the next game, falling behind 0:40. He responded with two excellent serves, followed by a composed volley at the net to level the score.
Sinner then created another break opportunity with a return right on the line. But the Czech, after a strong serve, rushed the net and saved the threat, eventually taking a 2-1 lead.
For the next several minutes, not much happened on court of note. Both players held serve comfortably, with Mensik continuing his aggressive play and matching the in-form Sinner shot for shot.
In the eighth game, the Czech earned the first break point of the match after a fantastic return of a drop shot, but narrowly missed his next return, allowing the Italian to level at 4-4 after deuce.
Mensik wasn’t rattled by the missed opportunity, kept serving superbly, and maintained a one-game lead. Sinner kept responding, and the opening set eventually went to a tie-break.
In the breaker, Mensik’s powerful first serve continued to be a weapon, and after an early mini-break thanks to an Italian error, he built a 6-3 lead. He then sealed the set with a brilliant down-the-line backhand on his first set point.
Mensik started the second set well, earning a break point in the opening game, but Sinner saved it with a strong serve. This time, it was the Czech who had to play catch-up, but he managed to keep pace. The Italian, however, stayed out of trouble and led 3-2.
Soon after, Mensik faltered on serve for the first time. His serve let him down, and after falling behind 0:30, he double-faulted twice in a row, gifting Sinner the break. The Italian consolidated the break after deuce, won the next game, and took the set 6-2, as Mensik's intensity dropped and his first serve percentage plummeted.
But at the start of the deciding set, Mensik came alive, ramped up his aggression, and immediately broke Sinner’s serve. The Prostejov native almost gave up his lead right away with some errors, but on Sinner’s break point, he handled a tough volley and then extended his lead to 2-0.
The Czech underdog didn’t create more break chances on return for a while, but it didn’t matter.
He held onto the early break advantage all the way to the end of this thrilling battle, sealing the win on Sinner’s serve and notching the biggest victory of his career against the world number two.
Although Mensik suffered several losses to lower-ranked players over the past year and dropped matches he was expected to win, he once again showed his ability to rise to the occasion against the world’s best.
With this win over Sinner, he improved his impressive record against top-five opponents (4-2) and top-10 players (9-8).
He also remains unbeaten at ATP tournaments this season. Doha is his second ATP event of the year, and in mid-January in Auckland, New Zealand, the reigning Miami Masters champion claimed his second career title. He now has 10 consecutive wins, including a run to the Australian Open round of 16, which he missed due to an abdominal injury.
It was in Doha two years ago that Mensik made his breakthrough on the main tour, reaching the final against Karen Khachanov and breaking into the Top 100 for the first time. Now, on his second appearance in the Qatari capital, he has a chance to go all the way to the final again.
If the afternoon session had gone according to the rankings, there could have been an all-Czech semi-final. However, Lehecka lost to Fils, so the Frenchman will be Mensik's next opponent.
The other semi-final will feature world number one Carlos Alcaraz and defending champion Andrey Rublev.