Novak Djokovic was deposed as world number one by Daniil Medvedev on Thursday after being knocked out of the Dubai ATP tournament by Czech qualifier Jiri Vesely.
Djokovic, who was playing in his first tournament of 2022 after being deported from Australia, lost 6-4, 7-6 (7/4) in his quarter-final tie.
US Open champion Medvedev, currently involved in the Acapulco tournament, becomes the third Russian man after Yevgeny Kafelnikov and Marat Safin to ascend to the world number one spot.
He will be the first man other than Djokovic, Roger Federer, Rafael Nadal, and Andy Murray to hold the top ranking since February 1, 2004.
Djokovic has been number one for a total of 361 weeks in his career.
Vesely, ranked 123 in the world and who had already defeated Djokovic at Monte Carlo in 2016, will face either Canadian sixth seed Denis Shapovalov or Ricardas Berankis of Lithuania for a place in the final.
"It's an amazing feeling. I never thought I would really have a chance against Novak, he's one of the greatest of all time, if not the best," said Vesely."After the last 12 months, I've been going through… it's unbelievable, I have so many emotions inside, it's hard to describe, it's just an amazing feeling."