Roger Federer has added yet another record to his vast collection when he became the oldest ATP No 1 by beating Dutchman Robin Haase 4-6 6-1 6-1 to reach the semi-finals in Rotterdam.
Needing to reach the last four to overtake great rival Rafael Nadal, Federer showed some early nerves in his quarter-final clash, dropping serve in the ninth game on his way to conceding the first set.
But the 36-year-old Swiss eventually began to punish a physically struggling Haase.
A demoralised Haase double-faulted on match point and after an initially restrained celebration Federer sat on his chair and looked close to tears as his feat sank in.
"Reaching number one is one of, if not the ultimate achievement in our sport," Federer said on court after being handed a huge No 1 shaped plaque by former Wimbledon champion Richard Krajicek, the tournament director.
"Sometimes at the beginning you just kind of get there because you played so well, but later you have to fight for it and have to wrestle it back from someone who deserves to be there. When you are older you maybe have to put double the work in. This maybe means the most to me in my career."
Federer, whose glittering career has re-ignited since taking six months off in 2016, has won three of the last five grand slams having gone five years without one.
One of the greatest ever late-career runs by any sportsman or woman means he surpasses Andre Agassi, who was the previous oldest man to top the ATP rankings at the age of 33 in 2003.
He was one of the first to congratulate Federer.
"Roger Federer continues to raise the bar in our sport. Congratulations on yet another remarkable achievement!!" Agassi said on Twitter.
Federer also set another record with the more than five-year gap between his previous and most recent stints as world number one being the longest since ATP rankings began in 1973.
He already holds the record of 302 weeks ranked number one.
Federer will face either Italian Andrea Seppi or young Russian Daniil Medvedev in the semi-final as he closes in on a 97th career title.