If someone had told Roger Federer 13 months ago that reclaiming the world number one ranking would require three more grand slam titles and three Masters 1000s, even he might have declared it mission impossible.
Fast forward a year and a bit, however, and the 36-year-old is back at the summit - the oldest man to make it since the ATP rankings came into being in 1973.
Few believed it was possible when he missed half of the 2016 season with knee and back problems.
But on Friday a 4-6 6-1 6-1 victory over Dutchman Robin Haase in the quarter-finals of the ABN AMRO tournament in Rotterdam guaranteed that Federer would rise to number one for the fourth time in his career.
Fourteen years after he first achieved it, this one felt extra special, said the 20-times grand slam champion.
"Well it's a deep sense of satisfaction," Federer said.
"A lot of work went into it, coming back from the injury obviously. Just having had the year that I've had, winning three slams, that's what it took to get the ranking.
"I think I've shown resilience. I have a great team around me and we took a lot of good decisions in the last three years. I always planned for longevity and I never gave up that I could get back to winning ways, without ever dreaming of world number again to be honest, that was too far."
Federer knew a run to the semi-finals in Rotterdam would knock Rafael Nadal off the top and make him the oldest player, man or woman, to be world number one.
Haase looked as though he had not read the script when he took the first set but Federer was never going to let the opportunity pass him by.
"I think it was great that I had to play for it this week," he said.
"I just didn't just get it by Rafa losing somewhere or dropping points or me sitting on the sidelines like in 2012 when I was on holiday and got the ranking two weeks after Wimbledon.
"This way to play a match where you know that you are going to be world number one if you win it, in front of a crowd, is a great feeling."
Incredibly Federer first became world number one in 2004 and on that occasion he stayed there for 237 weeks.
"The goal (this time) was to be world number one for a week, that's plenty for me," he said. "If it's more, great, I'll take it. If I play well, good things will happen.
"It's the ultimate achievement in our sport to get the number one ranking, it just doesn't come easy."
Federer now faces Italian Andreas Seppi in the semi-finals in Rotterdam after Seppi beat Russian Daniil Medvedev, while the other semi-final will feature Bulgaria's Grigor Dimitrov and Belgian David Goffin.
