Novak Djokovic suffered a shock 6-4 7-6(4) defeat by qualifier Jiri Vesely in the Dubai Tennis Championships quarter-finals on Thursday and will lose his status as world number one next week.
U.S. Open champion Daniil Medvedev, who is currently playing in a tournament in Mexico, will move to the top of the rankings on Monday.
Russian Medvedev will become the first player other than Djokovic, Roger Federer, Rafa Nadal or Andy Murray to top the rankings since Feb. 1, 2004.
Djokovic spent a record 361 weeks atop the rankings and won eight tour-level titles, including three Grand Slams, since regaining his top-ranked status in February, 2020.
"Unfortunately, it wasn't my day," Djokovic told reporters. "I congratulate Jiri. He played better. He just went for his shots. His serve was big. His whole game was big.
"I expected myself to play on a higher level, for sure. I mean, I can do better. But credit to Jiri."
Playing in his first event since being deported from Australia ahead of the year's first Grand Slam tournament, Djokovic lost serve in the opening game as Vesely matched him stroke for stroke.
Djokovic broke back at 1-2 but was never allowed to settle by Vesely, whose clever drop shots had his opponent scrambling at the net as the Czech sealed the first set 6-4.
The 20-times major winner started strongly in the second set but Vesely maintained his high level, racing into an early lead in the tiebreak to clinch his second career victory over Djokovic.
Djokovic said he was still trying to find his rhythm.
"The more matches I play, the more confident I will be, the more comfortable I'll be feeling on the court," the 34-year-old said.
Vesely said defeating Djokovic was an "amazing feeling".
"I never really thought I would have a chance against Novak," Vesely said. "He is one of the greatest of all time.
"It is great for tennis to have someone new at world No. 1. Novak is such a champion, he has been No. 1 for 361 weeks, but tennis needs new world No. 1s and there is a new generation coming up and I think it is great."
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