Daniil Medvedev will become the new world No.1 after Novak Djokovic was beaten by Jiri Vesely at the Dubai Tennis Championships.
Medvedev entered this week knowing victory at the Mexican Open would see him move to the top of the ATP rankings for the first time.
But the Russian was still waiting to play his quarter-final on Thursday when his ascension was confirmed courtesy of Djokovicâ€™s last-eight defeat in Dubai.
Djokovic, a seven-time year-end number one, had led the rankings for 86 consecutive weeks, boosting his record total to 361 weeks.
In his first tournament since missing the Australian Open, however, the five-time Dubai champion struggled to match 123rd-ranked Vesely and went down 6-4 7-6 (7-4).
Djokovic might have known what to expect, having lost his only prior meeting with Vesely in Monte Carlo in 2016.
The Serbian looked like getting to grips with his opponent at the start of the second set, applying pressure, but was then broken again.
He had to respond strongly to reach a tie-break.
Again, Vesely led, and Djokovicâ€™s match was summed up by a double-fault immediately after winning back the mini-break, allowing the Czech to prevail.
The victor hailed the end of Djokovicâ€™s reign, saying in his on-court interview: â€œItâ€™s great for tennis to have somebody new as world number one again.
â€œWe all know Novak missed the Australian Open, but heâ€™s been such a champion, as world number one for 361 weeks.
â€œTennis needs new number ones, new generations coming up. So, I think itâ€™s just great.â€