The 15-time Grand Slam winner will replace top-ranked Victoria Azarenka in next week's rankings after coming from a set down to beat former Wimbledon champion Petra Kvitova 3-6, 6-3, 7-5 on Friday in the quarterfinals of the Qatar Open.
It marks the end of an emotional two-and-half year journey for Williams marked by a string of injuries and doubts about whether she could dominate the women's game again. After clinching the victory with an ace, Williams held up one finger and wiped away tears as she addressed the cheering crowd.
"I never thought I would be here again," she said. "Oh, my gosh, I've been through so much. I never thought I would be here."
The 31-year-old Williams is the oldest player to attain the top spot, breaking the mark previously held by Chris Evert, who was No. 1 in 1985 just shy of her 31st birthday.
Regaining the top spot was not a priority two years ago for Williams. Simply returning to the court was a challenge.
Shortly after winning Wimbledon in 2010, she cut her feet on broken glass while on her way out of a restaurant in Germany. That led to two operations on her right foot. Later, she had clots in her lungs and needed to inject herself with a blood thinner. Those shots led to a pool of blood gathering under her stomach's skin, requiring another hospital procedure.
Williams lost the top spot to Caroline Wozniacki in 2010 and tumbled down the rankings. But she made a comeback worthy of a No. 1 in 2012, winning Wimbledon, the Olympic gold medal, the U.S. Open and the season-ending WTA championships. She lost only one match in 2012 after her first-round exit at the French Open.
"There were often times that I never thought I'd play again. Then I thought I'd never be able to win tournaments or Grand Slams," Williams said. "No. 1 was so far off. It was always a dream, but I was No. 1 when tragedy struck, and it was just an awful thing to happen. So I'm happy that I'm back."
Among those to pay tribute to Williams again becoming No. 1 was Maria Sharapova, the American's next opponent, who said it proved the staying power of the game's older players.
"It just shows you how the generations have shifted a bit," Sharapova said. "Maybe five, 10 years ago you saw a lot of the younger players achieving much more success and winning Grand Slams. And now you see Li Na and Serena doing so well ... It just shows you how if you stay healthy and you're able to keep working hard, the things that you're able to achieve in tennis at that age."
Kvitova, who has never beaten Williams, called her a deserving No. 1.
"She played so well last season. She won almost everything," she said. "That just took some time to be No. 1 for her and she is right now, and she deserves it."
It wasn't a dominating performance Friday. Erratic at times and appearing to favor a right ankle she injured at the Australian Open, Williams committed 31 unforced errors and hit eight double-faults.
She also struggled early on with the Czech's serve and forehand in the first set. But her own serve saved her, especially in the second set when she went on to win 13 of the final 14 points, pumping her fist several times and screaming — the first signs of emotion to be seen from her thus far in the tournament.
Down 4-1 in the deciding set, Williams broke Kvitova when she double-faulted to make it 4-3. Though it seemed the match would go to a tiebreaker, Williams broke with a wicked crosscourt backhand to make is 6-5 and won it with one of her 14 aces.
"I stuck in there, and I just never gave up. I always felt like if I can just keep fighting and I can just try to do it," Williams said. "I definitely did not play great. With that being said, Petra played unbelievable, and she was just hitting shots I had no chance to get; I don't think anyone on this tour could have gotten."