The pullout of the world number two leaves Rafael Nadal as the sole top five player in a depleted men's field: world number one Andy Murray, Novak Djokovic and Stan Wawrinka - all injured - are also missing from the last major tune-up prior to the August 28 start of the US Open.

"I am very sorry to pull out of the Western & Southern Open as I always enjoy playing here," Federer, a seven-time champion, said. "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." Federer put in a spectacular showing in Montreal, reaching the final in his first event since winning a record eighth Wimbledon title. But the 36-year-old was not himself in the title match against German Alexander Zverev, who secured a 6-3, 6-4 victory to tie the Swiss on five ATP titles apiece this season.

Federer's withdrawal assures Rafael Nadal a return to the number one ranking. Had Federer played, he would have had to reach the semi-finals as well as progress farther than his Spanish rival. Nadal, who has spent 141 total weeks in the top spot, will be back at number one for the first time since July 6, 2014.