The Spaniard will regain top spot if he reaches the final regardless of what current No 1 Novak Djokovic does in Beijing.
Djokovic, who needs to retain the title to have any chance of holding Nadal at bay, responded with a thumping 6-1, 6-2 defeat of American Sam Querrey to set up a semi-final against Frenchman Richard Gasquet.
Nadal, who enlarged his grand slam singles haul to 13 this year by winning the French and US Opens, has not been ranked one in the world since July 2011 but has produced a stunning run since returning from a long lay-off in February.
Fognini, ranked 19, nearly put a spanner in the works though before Nadal earned a 2-6, 6-4, 6-1 victory to reach the semis. “It’s (the number one ranking) something that doesn’t create more pressure for me,” Nadal, who will face fourth seed Tomas Berdych in the last four, said. “It’s something that already happened in the past. If that happens again, good, it will be special for me, but we’ll see.
“I am going to have a very tough opponent in front of me. It is not the moment to think about No 1... it is the moment to think about Berdych.”
Fognini was a point away from taking a 5-1 lead in the second set and an upset was well on the cards with Nadal seemingly troubled by a left-knee injury he suffered early in the match. However, he found his form in the nick of time and the Italian managed just one more game.
The stylish Fognini dished out some brilliant groundstrokes in the opening set to break Nadal three times. He continued his domination in the second set as he built a 4-1 lead but faltered with victory in sight. Nadal blazed back to win the next five games and the 27-year-old strolled through the decider to stay on course for an 11th title of an incredible season.
Meanwhile, Serena Williams brushed aside former world No 1 Caroline Wozniacki 6-1, 6-4 to advance to the semi-finals of the women’s event.
The top-seeded American broke Wozniacki twice to breeze through the first set in 26 minutes. Williams continued her dominance in the second set, breaking Wozniacki in the first and third games to go 4-0 up. The Dane fought back, closing the gap to 4-3, but it was not enough to stop Williams from winning the set 6-4.