Roger Federer creating the record of becoming the oldest world number one will be an "unbreakable" record, world number five Grigor Dimitrov said hours before Federer claimed the top spot.

The Swiss maestro achieved the feat following his three-set win over Robin Haase in the quarter-finals of the Rotterdam Open on Friday (16 February). He is currently 25 points ahead of previous incumbent Rafael Nadal.

Federer's return to the top of the world rankings for the first time since November 2012 is the culmination of an incredible 12 months, which has seen him rise from world number 18 to the top of the rankings.

The 36-year-old started 2017 after a six month absence due to a knee injury and immediately displayed a new aggressive brand of tennis. This saw Federer win seven titles last year including two Grand Slams and finish the year as the world number two.

The Swiss ace has carried his form into 2018 defending his crown at the Australian Open in January and now entering the semi-finals at the Rotterdam Open. He is unbeaten in 2018 and could extend his lead over Nadal to 345 points if he wins the event in the Netherlands.

"There aren't concepts or words to describe what he is doing," Dimitrov said talking about Federer's recent success, as quoted by the Express. "Every time he surprises, every time he achieves highs that seemed to be impossible.

"If he became No 1, it will be an unbreakable ever record," the Bulgarian world number five added.

Dimitrov has come up against Federer on six occasions in his career, but is yet to find success. The duo could again face each other in the finals in Rotterdam with the Bulgarian due to face David Goffin in the last four clash on Saturday (17 February).

Roger Federer
Federer and Dimitrov could face each other for the seventh time if they both win their semi-final matches at the Rotterdam Open Getty