Top seed Alcaraz dropped the first set before recovering to level, and then led by a break in the decider, but world number 25 Lehecka won the final four games for a 6-3, 3-6, 6-4 victory.

Alcaraz entered the clash on a seven-match winning streak following his triumph in Rotterdam earlier this month but ran into trouble against a confident Lehecka, who won his second career title in Brisbane to start the year.

"Honestly, I don't know what I could have done better," said Alcaraz. "I have to give credit to him as well, because when he was down, especially in the third set, he didn't give up.

"He was returning pretty well, really aggressively, no mistakes, or almost no mistakes."

The 23-year-old Lehecka will play British eighth seed Jack Draper on Friday for a place in the final. The Czech is through to the last four in Doha for the second time.

"I feel great," said Lehecka. "To win a match like that against a player like this is a super big achievement. I believed in myself and knew I had the level to produce this kind of tennis.

"I think the most important part was I believed in my game and did not back down."

Draper battled back to beat Matteo Berrettini 4-6, 6-4, 6-3 in the last of the quarter-finals.

Berrettini, a former world number six and Wimbledon runner-up, had knocked out Novak Djokovic in the second round.

Andrey Rublev converted his eighth match point against De Minaur to take his place in the semi-finals.

The Russian, a Qatar winner in 2020, had to dig deep to down the Australian 6-1, 3-6, 7-6 (10/8).

Fifth-seeded Rublev let slip a 5-2 lead in the final set as De Minaur scrambled back into contention to force a tiebreak.

Rublev finally got over the line, the 27-year-old saving a match point himself in a memorable quarter-final clash.

"When I had my first match point, I played really well," said Rublev.

"I did everything amazing and he just played an unreal rally. He made the backhand down the line and I played a good volley.

"Then he makes a forehand pass down the line. In most of the matches, normally after these points, they win - it's a turning (point).

"I then started to get a bit more tight and more emotional, I was a bit unlucky. Then when I lost my serve, I said 'Okay, whatever, if I'm gonna lose, at least just try to do your best until the end'."

This was his first win against a top-10 player since August last year.

Rublev's reward for his fourth semi-final appearance in Qatar is a meeting with Felix Auger-Aliassime.

The Canadian was leading his quarter-final against Daniil Medvedev 6-3 when the Russian had to retire.