Roger Federer became the oldest holder of the top spot in the new ATP world ranking after Rafael Nadal dropped to the second spot following his defeat to Dominic Thiem in the Madrid Masters quarterfinals on Friday.
The Swiss is missing the clay-court swing but is due to return on the grass at the Mercedes Cup in Stuttgart on June 11.
Rafael Nadal dropped down the rankings while defending a huge number of points he won last year when he claimed titles in Monte Carlo, Barcelona, Madrid and the French Open, to enable him to retain the position.
Nadal will have the opportunity to reclaim the top spot before the grass-court season if he wins the Rome Masters this week and an 11th French Open, which begins on May 27 in Paris.
He will face Fernando Verdasco or Damir Dzumhur in his opening match in the last 32 at the Rome Masters.
However, former world number one Novak Djokovic has dropped out of the top 15 to 18th for the first time since January 2007.
Having undergone elbow surgery after losing to Chung Hyeon in the Australian Open last 16 in January, Djokovic, a 12-time Grand Slam winner has suffered a series of early defeats in his return to action with the last 16 in Monte Carlo, his best run in five tournaments over the past three months.