Roger Federer has pulled out of the Cincinnati Open with a back injury, meaning Rafael Nadal will be the new world No.1 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.
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Nadal has spent 141 weeks at No.1 since first ascending there after reaching the Western and Southern semi-finals in 2008.
"It's been tough to get back to No.1," Nadal said.
"I'm happy to have the chance to be in that position."
Nadal will get his Cincinnati campaign underway against veteran Frenchman Richard Gasquet in the second round.
Gasquet accounted for Australia's John-Patrick Smith 6-4 6-4 in his tournament opener on Monday but the win was far from what the regulation scoreline suggested.
Smith, who qualified for the tournament, roared to an early lead in the second set as he evoked memories of Pat Rafter, chipping and charging at will to break Gasquet and take a 4-2 lead in the second.
It wasn't to last though. Gasquet broke back immediately but needed five chances to do so before breaking again and then serving out the match.
Croatia's Ivo Karlovic took a bit longer to get through his first round match.
He took down fellow big man Jiri Vesely 6-3 3-6 7-5 and served 29 aces on his way to a second round clash with Frenchman Jo-Wilfired Tsonga.
Also advancing in straight sets were Fabio Fognini, a 7-6 (7-5) 6-4 winner over Daniil Medvedev; qualifier Mitchell Krueger beat Benoit Paire 6-2 6-1 and Spain's Feliciano Lopez prevailed 7-6 (7-5) 6-1 over Korea's Hyeon Chung.
