Roger Federer spoke following his loss to Alexander Zverev in the Rogers Cup final. (1:13)
Roger Federer will not play in the Western & Southern Open, as representatives of the Cincinnati-based tournament announced Monday that he is unable to play due to a back injury.
"I always enjoy playing here," Federer said in a statement. "Cincinnati has some of the best fans in the world and I am sorry I will miss them."
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.
Roger Federer is skipping the Western & Southern Open this week due to a back injury. EPA/Nic Bothma
Federer, the Australian Open and Wimbledon champion this year, lost in the Rogers Cup final Sunday to Alexander Zverev. After the final, Federer said he was dealing with muscle pain after playing the hard-court tournament.
A seven-time champion in Cincinnati, Federer will be replaced in the draw by Thomas Fabbiano.
He is another high-profile tennis star to drop out of the Cincinnati tournament, along with Andy Murray and Marin Cilic. Murray is dealing with a hip injury, while Cilic has an adductor injury that he suffered at Wimbledon.
On the women's side, Victoria Azarenka and Maria Sharapova have announced that they will not play in the event.
They will all turn their focus to the US Open, which begins Aug. 28.
Information from The Associated Press was used in this report.