Rafael Nadal described his return to world number one as "something special" after his resurgence following injury problems.
The Spaniard was last at the summit of the men's game in July 2014, but Roger Federer's absence from this week's Western & Southern Open in Cincinnati with a back injury means Nadal will usurp Andy Murray - also absent with a persistent rib problem - atop the rankings.
It is a particularly sweet moment for Nadal, who has had well-documented fitness issues and missed the end of last season due to a wrist complaint.
Nadal, who won a 10th French Open in June, expressed sympathy for the stricken Federer, but is delighted to be back on top.
Speaking at a news conference, the 31-year-old said: "Obviously it's bad news for the event that Roger is not playing.
"[But] for me personally obviously to be back to that position is something special.
"A lot of things have happened since the last time I was in this spot. Injuries and some tough moments, of course, but I have held the passion and the love for the game, and that's why I have the chance to be back in that position again.
"So I'm just trying to enjoy the moment and just trying to be ready to compete well here. That's the most important thing for me now."
Nadal faces Richard Gasquet on Wednesday.