Serb Novak Djokovic, who held the top honour for 101 weeks, will slide down to second.
"It would be great if I could finish the season on number one," said Nadal. "But for sure I need to win more matches if I want to be there."
On Saturday, he played Tomas Berdych of the Czech Republic in the semifinal match of the China Open tennis tournament at the National Tennis Stadium in Beijing.
Nadal was a break up at 4-2 in the opening set of their semifinal match when Berdych retired soon after taking an injury timeout.