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).