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 victory.