Roger Federer says becoming the oldest world number one in tennis history - 14 years after he first topped the rankings - is a "dream come true".
The 36-year-old beat Robin Haase 4-6 6-1 6-1 in the last eight of the Rotterdam Open to replace Rafael Nadal, 31, at the top of the standings.
Federer surpassed Serena Williams, who was 35 when she lost her women's number one ranking in May 2017.
"This one means the most at 36 years of age, almost 37," the Swiss said.
"When you're older you have to work double the amount. You have to wrestle it back from someone who's worked hard to get there.
"Reaching number one is the ultimate achievement in tennis. It's been an amazing journey and to clinch it here, where I got my first wildcard in 1998, means so much."
Eight-time Grand Slam winner Andre Agassi previously held the men's ranking record - he was 33 when he lost the number one spot in September 2003.
Federer's hopes of reaching the semi-finals in the Netherlands looked slim when he lost the first set, but he dropped just two further games to set up a tie with Italian Andreas Seppi or Russia's Daniil Medvedev.
At the end of the match, the 20-time Grand Slam champion sat down and wiped away tears, before returning to the court to take the acclaim from the crowd and receive a trophy for his achievement.
He first became world number one in February 2004, but has not topped the rankings since October 2012 and slipped to a low of 17th in January 2017.
That was after he spent six months out recovering from an operation on a knee problem.
However, he has since won eight titles, including Wimbledon last year and two Australian Opens.
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Six-time Grand Slam winner Stefan Edberg on BBC Radio 5 live
The way Federer plays, the way he moves on the court, he is probably the only one who was going to get to number one at this stage, this late in his career.
I doubt there is going to be somebody in the near future doing the same. He has done an extraordinary job. It is quite incredible, I don't think he believes it himself, and I think everybody is surprised.
He can play freely now, because whatever he does now is going to be a bonus. Hopefully he can keep the momentum going, and if he keeps being healthy he is got a really good shot at Wimbledon again.
Federer in numbers
- When the ATP rankings are updated on Monday, it will be five years and 106 days since he was last at the top - the longest gap between stints.
- He first reached the summit 14 years and 17 days ago in February 2004 - the longest gap between first and most recent spells.
- The Swiss is only 13 behind Jimmy Connors' record of 109 tour-level titles