Rafael Nadal is set to return to the top spot in the world rankings, following the news that Roger Federer will miss this week's Cincinnati Masters.
Federer, who was beaten in the Montreal final on Sunday night, has pulled out of Cincinnati due to a back injury.
And that means French Open champion Nadal will take over from Britain's Andy Murray atop the world rankings next Monday.
Murray had already been ruled out of the Cincinnati event, which meant Federer and Nadal, who have both enjoyed renaissances this year, were set to duel for the world No 1 spot.
But the withdrawal of Australian Open and Wimbledon champion Federer means the 31-year-old Nadal will top the rankings for the first time since July 6, 2014.
Federer said in a statement: "I am very sorry to pull out of the Western and Southern Open, as I always enjoy playing here.
"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."
Murray is continuing his recuperation from a hip injury and was guaranteed to lose his top ranking this week, having reached the Cincinnati final last year.
And ahead of the US Open, which gets underway on 28 August, Rafael Nadal is set to be the only member of the world's top six taking part in this week's tournament.
Federer joins Murray, last year's winner Marin Cilic, Novak Djokovic and Stan Wawrinka on the sidelines.
Nadal could have reclaimed the No 1 spot by reaching the semi-finals in Montreal last week, but was beaten by Denis Shapovalov in the third round.
The Spaniard, a 15-time Grand Slam winner, has spent 141 total weeks in the top spot, and became World No 1 for the first time in August 2008 after a semi-final run at Cincinnati.