Nadal only needed to make the final in Beijing to edge ahead of Novak Djokovic in the new rankings, which will be announced on Monday.
The Spaniard was a break up at 4-2 in the opening set when Czech Berdych retired soon after taking an injury timeout.
Nadal has enjoyed a remarkable season, returning from injury to dominate the hardcourt tournaments with a win-loss record of 27-0.
Djokovic, who has topped the rankings for 101 weeks, plays Richard Gasquet of France in the other semi-final later on Saturday.
Nadal, sidelined for the second half of last year with a knee injury, has finished the season as No.1 twice before, in 2008 and 2010, and is unbeaten since crashing out of Wimbledon in the first round in June.
The 27-year-old, tipped to eclipse Roger Federer's record of 17 grand slam titles, has won 10 ATP World Tour titles in 2013.
In the women's semis, top-ranked Serena Williams will play Agnieszka Radwanska of Poland and Petra Kvitova of the Czech Republic takes on Serbia's Jelena Jankovic.