Nadal and Federer, the current world number one and three respectively, have been the two best players on tour this season. They are ranked one and two in the ATP Race to London owing to their combined nine titles in 2017.
The Spaniard has played in seven finals and won four titles including an unprecedented 10th title at Roland Garros. Andy Murray's withdrawal from the Rogers Cup and the Cincinnati Masters has seen Nadal take over the top spot in the men's singles ranking – the first time since 6 July 2014.
The Swiss ace has been a little more clinical than his long-time rival and has won 35 of his 38 matches this season. He has captured five titles since January, which includes the Australian Open and a record breaking eighth Wimbledon title.
Henman admits that it is 'naïve' to be surprised by the achievements of Nadal and Federer this season and despite the US Open being the Spaniard's least favourite Grand Slam, he believes the two veterans are the outright favourites going into the tournament.
"I didn't necessarily think he'd get back to number one but I never write these true greats of the game off," Henman said about Nadal's rise to world number one, as quoted on Sky Sports.
"He's only 31 and the way he's played this year, the rankings don't lie, he's been absolutely phenomenal. He thoroughly deserves it and he's such a great ambassador for the game.
"He's won in New York before. I think this has been probably his toughest event, the ball normally is a little bit quicker and a bit tougher for him to control with how much spin he puts on, but he's definitely playing well enough and will have the confidence and the knowledge, and, for me, he goes in as the favourite along with Federer," the former world number four explained.
"For Roger to play seven, best-of-five-sets matches will be a challenge for his body but he seemed to overcome it pretty well in Melbourne. They're just incredible and I think it's naive to be surprised by them," he added.