Roger Federer has withdrawn from the Western & Southern Open in Cincinnati due to a back injury.
"I always enjoy playing here," Federer said in a statement on Monday. "Cincinnati has some of the best fans in the world and I am sorry I will miss them. Unfortunately, I tweaked my back in Montreal and I need to rest this week."
Thomas Fabbiano of Italy will enter the Cincinnati tournament in place of Federer, who admitted after his loss to Alexander Zverev at the Rogers Cup on Sunday that he was dealing with muscle pain.
Federer's withdrawal allows Spaniard Rafael Nadal to return to No. 1 when the new ATP rankings are released Aug. 21. Nadal last held that spot on July 6, 2014.