Roger Federer moved to within one victory of becoming the oldest man to take the world No 1 ranking on Thursday with a hard-fought 7-6 (10/8), 7-5 victory over Germany’s Philipp Kohlschreiber at the Rotterdam Open.

The 36-year-old will now face Robin Haase in the quarter-finals on Friday.

“Tonight was complicated, I had to fight and struggle,” said Federer. “I had a good game plan going in, but I was never able to completely pull it off.

“He came out with a clean start. I was serving well, that’s a great sign. I’d rather do that and return poorly.”

Thursday’s win was Federer’s 13th in as many meetings against Kohlschreiber.

The first set was a 54-minute struggle with the top seed having to work hard against the veteran German who was broken for the only time in the match when he double-faulted to hand Federer a 6-5 lead in the second set.

Federer, the 20-time Grand Slam title winner, then eased to victory in the 12th game with a volley winner on second match point.

The Swiss is excited about the possibility of returning to the top of the rankings 19 years after he played Rotterdam in 1999 on his first career wild card.

“This is an exciting challenge, I’ve struggled to try and get there. I had to win a lot of matches last year,” said Federer, who was world No 1 for the first time in February 2004.

However, he was last at the summit in October 2012 and slumped to 17 in the world in January last year.

“I never imagined this after my [February 2016] knee surgery. Number one is a tough place to get to.

“The most important thing is to be healthy, I would have had great regrets if I had not come here this week. I’m very excited for tomorrow’s match, I can’t wait for it to come around.”

Second seed Grigor Dimitrov outlasted Serbia’s Filip Krajinovic 7-6 (7/4), 7-5 to also reach the quarter-finals.

Dimitrov has now reached a second straight quarter-final in the Dutch city, advancing with 13 aces and saving all four break points he faced.

The Bulgarian will face Andrey Rublev on Friday after the 20-year-old Russian struggled for nearly two hours before finally seeing off Damir Dzumhur 6-4, 7-6 (7/4).

Rublev, the world number 34, forced his Bosnian opponent to save 10 of 15 break points while he saved nine of 13.

Russian Daniil Medvedev joined his compatriot in the last eight by beating Pierre-Hugues Herbert of France 3-6, 7-6 (7/2), 6-4.

The two-and-a-quarter-hour contest featured 13 aces from the loser and just four from the winner.