Rafael Nadal will return to number one in the world rankings after his semi-final opponent Tomas Berdych retired injured at the China Open Saturday, capping an astonishing comeback for the 13-time Grand Slam champion.

The Spaniard needed to make the final in Beijing to edge ahead of Novak Djokovic in the new rankings, which will be announced by the ATP on Monday.

The second seed will now confront Djokovic on Sunday in the climax of the tournament, after the defending champion powered his way though his semi-final, dumping out France's Richard Gasquet 6-4, 6-2.

The Serb, who is on his 101st week as the top-ranked player, will be unseated by Nadal even if he reclaims the title.

Nadal, who last topped the rankings in July 2011, has enjoyed a remarkable season, returning from injury to dominate the hardcourt tournaments.

He told reporters after his match that returning to the top spot had capped "a great year, one of the best years of my career without any doubt".

"Sure is special be back to the top position of the ranking after more than a half year without playing tennis," he added.

"I felt during all the season very excited every time I go on court."

Djokovic also praised Nadal's performances this season, saying he was "looking forward to the challenge" of the final.

"I congratulate him on a big season. He has definitely been playing the best tennis in 2013, but the season is still not over," he said, adding that he needed to "step up and try to play my best game".

Berdych pulled out of his semi-final after receiving treatment for an injury with Nadal leading 4-2 in the first set.

Nadal had broken the Czech's serve in the fifth game after the pair appeared to be matching each other competitively in the early stages.

The world number five told reporters after the match that he suffered a back injury when he reached for a shot.

Djokovic has been in dominating form in China, where he has an unbeaten run stretching back to 2009. He has won three of the four last tournaments, and did not attend in 2011 due to injury.

Gasquet took an early 3-1 lead, but rarely threatened the top seed, succumbing in one hour 20 minutes.

The high point of the game for the world number 10 was when he won a point in the fifth game of the second set by hitting a return through his legs.

Nadal was sidelined for the second half of last year with a knee injury and is unbeaten since he crashed out of Wimbledon in the first round in June.

He has twice finished the season as the top ranked player, more recently in 2010, and the first time in 2008, after he edged into the top position by winning gold at the 2008 Beijing Olympics.

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 Saturday's women's semi-finals, world number one Serena Williams powered through her match with Poland's Agnieszka Radwanska 6-2, 6-2.

The American 32-year-old pulled up injured after appearing uncomfortable in the fourth game of the third set, clutching her back as she walked off court with medical officials.

But the brief pause did not result in Williams losing her momentum, as she ruthlessly finished off the third seed in convincing style, breaking serve in the next game before closing in on victory.

The top seed in Beijing said she had not picked up any serious injuries during the match.

"My back was a little tight from playing a lot so I was just trying to release it," she said.

Williams will face eighth seed Jelena Jankovic in Sunday's final after the Serb defeated Czech Petra Kvitova following an early scare.

Jankovic lost the first set on a tie-break, but the world number 11 dominated the following sets, losing only two games to crush Kvitova 6-7 (7/9), 6-1, 6-1.