Roger Federer has pulled out of the Cincinnati Open with a back injury, meaning Rafael Nadal will be the new world number one in next week's rankings.

Federer, who has participated in the Ohio warm-up tournament for the US Open on seven occasions, said he had picked up the injury in Montreal, where he lost to Germany's Alexander Zverev in the final.

"I am very sorry to pull out of the Western and Southern Open as I always enjoy playing here," Federer said in a statement issued by the tournament.

"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's withdrawal assures that Nadal will be top of the rankings when they are issued on August 21. The Spaniard will return to the top spot for the first time since July 6, 2014.

Only Federer could have prevented the Spaniard from returning to the summit with British number one Andy Murray guaranteed to be toppled as he continues his recuperation from a hip problem.

 The Scot, who lost to Marin Cilic in the 2016 Cincinnati final, will lose 600 points, meaning Nadal would usurp him even if the 31-year-old followed his big-name rivals in withdrawing from the tournament.

Nadal is the only player in the world's top six still standing in the Cincinnati event, with Federer now joining Murray, Cilic, Novak Djokovic and Stan Wawrinka on the sidelines.

Nadal could have reclaimed the world number one spot by reaching the semi-finals in Montreal last week, but fell to Denis Shapovalov in the third round.