Peter Fleming has revealed how Roger Federer used his serve to outfox opponents over the years, something that his competitors have failed to do. The Swiss learned that serving at 120 kmph was fast enough for the opponent to have trouble hitting the ball cleanly, which allowed him to focus more on his accuracy.

The 36-year-old became the oldest world number one in tennis following his quarter-final victory over Robin Haase at the Rotterdam Open on Friday (16 February). He broke Andre Agassi's long standing record – the American was 33 when he reached the summit in 2003.

The feat was made possible as he won his 20th Grand Slam title at the Australian Open, beating Marin Cilic in an engrossing contest lasting five sets.

Federer has now won three of the last five majors and victory in Melbourne was the sixth Australian Open victory of his career. He becomes only the fourth player after Margaret Court, Serena Williams and Steffi Graf to win 20 or more major singles titles.

The Swiss tennis legend will now look to win at Rotterdam, having earlier won the event on two occasions in 2005 and 2012. He is making his ninth appearance this time around. He will face Grigor Dimitrov in the final.

Fleming believes that Federer's understanding of the game at an early age helped him to go on to win many titles.

"Roger learned very early, at least in the first year or two of being a pro, that 120mph hit accurately is plenty powerful enough," Fleming said, as quoted by the Express. "So many players come out thinking they have to hit 130, 140, otherwise they're going to lose the advantage. And by contorting their body to try and get that extra power, they lose accuracy."

Roger Federer
Roger Federer became the oldest world number one in tennis following his win over Haase at the Rotterdam Open on Friday (16 February) Getty