Wimbledon champion Roger Federer has withdrawn from the Western & Southern Open, which he has won seven times.
Tournament officials announced Federer's withdrawal on Monday, the first full day of matches at the event in Cincinnati.
The 19-time major champion says he ''tweaked'' his back last week at the Rogers Cup, where he lost Sunday to Alexander Zverev in the final.
The Swiss star, No. 3 in the ATP Tour rankings, becomes the fifth of the top six players to skip Cincinnati because of injuries. He joins top-ranked Andy Murray, No. 4 Stan Wawrinka, fifth-ranked Novak Djokovic and sixth-ranked Marin Cilic, the defending champion.
Federer's withdrawal means Rafael Nadal will return to No. 1 when new ATP rankings are released on Aug. 21. The Spaniard will be No. 1 for the first time since July 6, 2014.
Nadal has spent 141 weeks as the men's No. 1 player.
(For more sports news and updates, follow Ahram Online Sports on Twitter at @AO_Sports and on Facebook at AhramOnlineSports.)