Serena Williams on Friday became the oldest woman to claim the world number one ranking when she reached the Qatar Open semi-finals with a 3-6, 6-3, 7-5 win over Petra Kvitova.
Williams, 31, will take over from Australian Open champion Victoria Azarenka as the world's best player on Monday, returning to the top spot for the first time in over two years.
Chris Evert at 30 was the previous oldest world number one.
Williams, who was on the verge of tears for large parts of the deciding set, was 4-1 down before reeling off six of the last seven games, taking the match with an ace down the middle.
"I'm so sensitive these days, I'm always crying, but I never thought I would be here again," said Williams, who was crying as she gave her post-match interview.
"When I was down I heard people cheering for me and I don't get that all the time. I never thought I would be number one again."
Williams will be the world number one for a 124th career week, a total only bettered by Steffi Graf (377), Martina Navratilova (332), Evert (260), Martina Hingis (209) and Monica Seles (178).
At 31 years, 4 months, 24 days, Williams becomes the oldest woman to hold the world No.1 ranking since the introduction of computer rankings in 1975.
Evert was 30 years, 11 months, 3 days when she last held the No.1 ranking the week ending Sunday, November 24, 1985.
It will be Williams' sixth spell at the top, nearly 11 years after she first claimed the mantle in July 2002.
"I am so thankful that I have the opportunity to get back to No.1. It has been a long road back and it's a great feeling. It has been a lot of hard work but I don't want to stop here," she added.
Williams began the 2013 season by claiming the title at Brisbane, followed by a disappointing, injury-hit quarter-final exit at the Australian Open.
The win in Brisbane improved her career title tally to 47.
A stellar 2012 season saw her claim her 14th and 15th Grand Slam titles at Wimbledon and the US Open, gold medals in the singles and doubles at the Olympics, and the title at the year-ending WTA Championships in Istanbul.
She finished the year with a 58-4 record.
Williams first reached the top ranking on July 8, 2002 at 20 years old, becoming the 12th woman to hold the top spot.
In September 2008, she returned to No.1 more than five years after first claiming the top spot.