Rafael Nadal will be the new world number one from next Monday after Roger Federer withdrew from the Cincinnati Masters with a back injury.
The Swiss 19-time Grand Slam winner was the only player who could have denied Spain's Nadal taking the top spot in the rankings from Andy Murray.
Federer was injured in Sunday's Rogers Cup final defeat by Alexander Zverev.
Briton Murray pulled out of Cincinnati last week as he continues to recover from a hip problem.
Nadal returns to the top of the rankings for the first time since July 2014.