World No 3 Carlos Alcaraz was left ruing a mistake that let Jiri Lehecka off the hook in the third set and allowed the Czech to fight back and claim a 6-3 3-6 6-4 victory in the Qatar Open quarterfinals on Thursday.
Four-time Grand Slam winner Alcaraz led 4-2 in the deciding set and was 40-30 up on Lehecka's serve when the top seed hit the net on a return to give his unseeded opponent the chance to hold.
Lehecka went on to win every remaining game and clinch his first victory over a top-three player.
“It was about one point. I think that's what makes tennis really difficult. One point makes a difference," Spaniard Alcaraz told reporters after his second loss of the year.
"At 4-2 break point, it was just a few centimetres down in the net, so it could turn the match completely. But then 4-3, he started playing great tennis.
"Probably I could have done a bit better from 4-3, but his game was incredible. He was returning pretty well, really aggressively, no mistakes, or almost no mistakes, so that's what I have to learn."
Alcaraz, who won Wimbledon and the French Open last year, said there was little he could have done differently to win the match.
"Next time, I just have to keep going when I am up in the match, with serves. But in my mind, I don't know what I could have done better, and I give credit to him," Alcaraz added.
Former Australian Open quarterfinalist Lehecka was elated after the win.
"For me 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," he said.
Lehecka faces Briton Jack Draper in the semifinals on Friday.