Rafael Nadal was assured a return to world number one for the first time since July of 2014 on Monday when 19-time Grand Slam champion Roger Federer withdrew from this week's ATP Cincinnati Masters tournament.
Third-ranked Federer said he injured his back in Montreal, where he lost in the final to Germany's Alexander Zverev.
Federer, this year's Wimbledon and Australian Open champion, was the only player who could have denied world number two Nadal the top spot in next Monday's rankings.
The 31-year-old Spaniard, who won his 15th Grand Slam title at this year's French Open, will overtake Britain's top-ranked Andy Murray, who withdrew from Montreal and Cincinnati with a hip injury.
Federer, a seven-time champion on the Cincinnati hardcourts, could yet move into the world number one spot, but his next chance will come at the US Open, the year's final Grand Slam event that begins August 28 in New York.
"I am very sorry to pull out of the Western and Southern Open as I always enjoy playing," Federer 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."
Nadal, who has spent 141 total weeks in the top spot, became world number one for the first time in August 2008 after a semi-final run at Cincinnati.
For Federer to have taken the top spot at Cincinnati, he would have had to at least reach the semi-finals. Had Federer reached the final, Nadal would have had to win the title to stop the Swiss star from becoming number one.
Federer will be replaced in the men's draw by Italian lucky loser Thomas Fabbiano, the world number 85 who will enjoy a first-round bye and face either Argentina's Diego Schwartzman or Russia's Karen Khachanov in the second round.