Carlos Alcaraz came from behind to beat Karen Khachanov on Thursday and book his spot in the semi-finals of the Qatar Open but it was the end of the road for Jannik Sinner as he lost in three sets to Jakub Mensik.
Playing his first tournament since completing his career Grand Slam at the Australian Open earlier in February, Alcaraz will next face Andrey Rublev for a spot in the final in Doha.
"It was a really close and tight match," Alcaraz said after his 6-7 (3/7), 6-4, 6-3 quarterfinal win.
"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.
"I'm just really proud about the way that I (fought)."
The first set rumbled towards a tie-break as neither Alcaraz nor Khachanov was able to force a breakthrough on the other's serve at the start of their quarterfinal clash.
Khachanov then raced into a 6-1 lead in the breaker before finally converting set-point at the third time of asking.
But seven-time Grand Slam winner Alcaraz forced his way back into the tie by breaking Khachanov to 15 in the fifth game of the second frame.
Alcaraz repeated the trick by again pouncing on the Russian's serve at 2-2 to get his nose ahead in the third set.
The world number one then broke the seventh seed in the ninth game to seal his spot in the last four in style.
There he will meet fifth seed Rublev after the Russian won his quarterfinal encounter with former world number three Stefanos Tsitsipas 6-3, 7-6 (7/2).
World number two Sinner went down 7-6 (7/3), 2-6, 6-3 to sixth seed Mensik as the Italian's comeback from Australian Open semi-final defeat came to an early end.
"Before the match I knew about Jannik's quality. A great guy, a great champion. Already what he has done in his young career, it's pretty impressive," 20-year-old Mensik said.
"But even with that thought, I came to the match to win it, and actually having a winning mindset."
Mensik will play Arthur Fils in the semifinals, following the Frenchman's straight-sets victory over Jiri Lehecka.