Novak Djokovic will lose his number one ranking after being beaten 6-4 7-6 (7-4) by Jiri Vesely in the quarter-finals of the Dubai Tennis Championships.
The tournament was Djokovic's first of 2022 after he was not allowed to compete in the Australian Open because he is unvaccinated against Covid-19.
The Serb will now be replaced by Daniil Medvedev at the top of the men's rankings on Monday.
"I never thought I would really have a chance against Novak, said Vesely.
The Czech Republic player, ranked 123 in the world, was broken when serving for the match, before winning the second set tie-break to complete an inspired victory.
"It's an amazing feeling," added Vesely, speaking to Amazon Prime. "He (Djokovic) is one of the greatest of all time if not the best."
Russia's Medvedev will be the first player other than Djokovic, Roger Federer, Rafael Nadal and Andy Murray to rise to the top of the rankings for 18 years.
Djokovic has spent a total of 361 weeks as world number one, which is a record in the men's game.
Vesely beat the 20-time Grand Slam champion in 2016 on the clay of Monte Carlo and repeated the feat on the hard courts of Dubai.
A Djokovic double fault on break point at 3-3 in the opening set gave the initiative to Vesely, who went on to serve out and take command of the match.