Roger Federer is delighted and a little surprised to see Rafael Nadal back at world number one after both men spent lengthy spells out of action due to injury last year.
Federer called time on his 2016 season after Wimbledon due to a knee problem, forcing him to miss the US Open and the Olympics.
Nadal then ended his year in October as he sought to recover from a wrist problem, but the two modern greats of the sport returned strongly in 2017.
They contested a nostalgic final at the Australian Open in January, a match won by Federer, who also triumphed at Wimbledon.
Nadal meanwhile reasserted his familiar dominance on the clay of Roland Garros, later adding the US Open to his 10th French Open crown in a throwback year for men's tennis.
It is Nadal who has climbed to the top of the ATP rankings in the absence of injured pair Andy Murray and Novak Djokovic, leaving Federer, speaking ahead of the new Laver Cup, to express his admiration for the achievement of his long-term rival.
He said: "Am I surprised that he's world number one right now? Maybe a little bit, just because last year we were... both injured, not knowing where our careers were going to go, but I'm not surprised that he came back and won the French Open and the US Open.
"Being number one is a whole different story. For that we had to see how Murray was going to play and Djokovic was going to have a word to say also, so we didn't know that they were going to get hurt and have bad seasons. But it's super impressive and I'm very happy for him."