Rafael Nadal will supplant Novak Djokovic atop the rankings after advancing to the final of the China Open on Saturday when Tomas Berdych of the Czech Republic retired with a back injury.
The Spaniard was a break up at 4-2 in the opening set of their semifinal match when Berdych retired soon after taking an injury timeout.
Nadal, who improved to 27-0 on hard courts this season, will return to the top ranking for the first time since July 2011. Djokovic has been No. 1 for 101 weeks.
"(This is) a great year, one of the best years of my career without any doubt," Nadal said. "Sure is special (to) be back to the top position of the rankings after more than a half year without playing tennis."
Nadal had been assured of reclaiming the top spot if he reached the final in Beijing or if Djokovic failed to defend his title.
The Serb plays Richard Gasquet of France in the other semifinal later Saturday.
Nadal, the reigning French Open and U.S. Open champion, has had a dominant year since returning in February from a seven-month layoff due to a left knee injury. He has won 10 titles and reached 12 finals overall, compiling a tour-best record of 65-3 on the season.
He hasn't lost a match since falling in the opening round at Wimbledon to 135th-ranked Steve Darcis of Belgium in June, his only major blip this year.
In the women's semifinals, top-ranked Serena Williams will play Agnieszka Radwanska of Poland and Petra Kvitova of the Czech Republic takes on Jelena Jankovic of Serbia.