Roger Federer has described his latest remarkable ascent to top of the ATP world rankings as the one that means the most to him.

The 36-year-old will become the oldest man to ever sit at the summit of the men's game on Monday after beating Robin Haase in three sets to reach the semi-finals of the Rotterdam Open, a run that will see him overtake old foe Rafael Nadal.

It is the crowning achievement of a stunning resurgence from the evergreen Federer, who was as low as 17th in the world in January 2017, having missed six months through injury in the previous year.

And Federer, now a four-time world number one, who holds the record for most weeks at the top with 302, believes this achievement outstrips the other times he held the title of best male player on the planet.

"I think reaching number one is one of the biggest, if not the ultimate, achievements in our sport," Federer, who was last number one in November 2012, said on court during a ceremony honouring his rise back to the top.

"Sometimes at the beginning you get there because you're playing so well, later you wrestle it back from someone else who deserves to be there, and when you're older you feel you maybe have to put sometimes double the work in."

"So, this one maybe means the most to me throughout my career, getting to number one and enjoying it at 36, almost 37, years old. It's a dream come true, I can't believe it."

Federer also, somewhat tongue-in-cheek, apologised to Haase, who had to sit through a farewell ceremony for the new number one's friend and retired compatriot Marco Chiudinelli in Basel last year.

"I'd also like to thank Robin for putting up a great fight, he's not been well all week," he added. "Ironically, he has to go through another ceremony with another Swiss guy, six months ago in Basel he retired my good friend Marco and now he has to watch this, I'm sorry you have to watch these things!"

Federer will return to world number one for the first time in over five years after the 36-year-old defeated Robin Haase to reach the semi-finals of the Rotterdam Open.

After knee surgery in 2016, Federer's career has gone from strength to strength despite his advancing years, while his closest rivals all continue to struggle.

Starting with an unexpected triumph at last year's Australian Open, Federer has added another three grand slam titles to his tally – taking it to 20 – but regaining his spot at the top of the rankings had eluded him until now.

Novak Djokovic, Andy Murray and Rafael Nadal have all sat above Federer in recent years as the 'fab four' have dominated the men's game.

However, with Djokovic (elbow) and Murray (hip) both spending a significant amount of time on the sidelines it has become a straight battle with Nadal for top spot.

Injuries have not escaped Nadal, either, opening the door for the evergreen Federer to reclaim the world number one spot – a position he has not occupied since November 4, 2012.

Nadal's 2018 Australian Open ended in retirement at the quarter-final stage, while Federer went on to retain the crown, enabling him to close within 155 points of the Spaniard in the rankings, and after accepting a wildcard in Rotterdam, there has been another changing of the guard due to his run in the Netherlands.

The achievement has come 14 years after he first became world number one, another prime example of Federer's longevity.

To emphasise that further, the top 10 Federer first reigned over looked like this….

1. Roger Federer
2. Juan Carlos Ferrero
3. Andy Roddick
4. Guillermo Coria
5. Andre Agassi
6. Rainer Schuttler
7. Carlos Moya
8. David Nalbandian
9. Mark Philippoussis
10. Paradorn Srichaphan

Not one of those names are still playing apart from Federer, although his long-term rival Nadal was beginning his own quest. Then just 17 years old, he ranked 40th.