Safina was forced to retire after only the second game of her match
Serena Williams will finish the year as world number one after Dinara Safina withdrew from the season-ending Sony Ericsson Championships in Doha.
Safina broke down with a back injury after two games of her opening round-robin game against Jelena Jankovic.
The Russian promptly withdrew from the event, handing Williams the top spot.
Wimbledon champion Williams later won her second match of the tournament, defeating her older sister Venus 5-7 6-4 7-6 (7-4).
Serena only had to win one more match than Russia's Safina to overtake her in the rankings and she had already beaten Svetlana Kuznetsova in her opening Maroon Group game on Tuesday.
"It's such a great feeling," said Serena on regaining the top spot. "Dinara is such a great player. She must be really hurt, because she never gives up."
Earlier on Wednesday, sixth seed Victoria Azarenka lost 1-6 6-4 7-5 to fourth seed Caroline Wozniacki.
I did everything possible to play here, but I could not handle this pain any more
Safina, who will be replaced in the White Group by compatriot and first alternate Vera Zvonareva, later revealed she would be sidelined for at least six weeks and could miss January's Australian Open.
She also said that she had been suffering with the problem for three months and that she needed cortisone injections to play in the tournament.
"My body just gave up," said the 23-year-old Safina. "I did everything possible to play here, but I could not handle this pain any more."
It was an unfortunate end to the season for Safina, who spent 25 weeks at the top of the rankings but whose lack of Grand Slam titles, next to Williams' 11 overall, left many questioning her number one credentials.
"I think I should have taken a break after the US Open but I was chasing this number one, so I was fighting with my body," she added.
Serena led Venus 4-1 in the third set but let her back into it and had to save a match point when serving at 6-5.
Serena then jumped into a 3-0 lead in the tie-breaker and she converted her first match point with an ace after two hours and 41 minutes of play.
It was defending champion Venus' second straight loss, but she can still reach the semi-finals if she beats Kuznetsova on Thursday and Serena defeats Elena Dementieva.
Denmark's Wozniacki saved a match point in the final set to see off Azarenka in their White Group encounter.
Azarenka was up a break four times in the third set but failed to finish things, letting her 19-year-old opponent break back each time.
Wozniacki, who made just two winners in the first set, saved a match point with Azarenka leading 5-4 in the third before finally holding serve in the decider.
The Dane broke again for 6-5 after the Belarussian - who had already been warned - was docked a point for smashing her racquet, and Wozniacki sealed victory with a service winner after two hours and 58 minutes.
After her win, Wozniacki, playing her first match in the event, seemed to be affected by the sore hamstring that forced her to retire from the Luxembourg Open last week.
The season finale is split into two groups of four, with the top two in each group making the semi-finals and the eventual champion taking home $1.55m (£944,000).