Nadal, the King of Clay, is on the hunt for a record-extending 12th title at Roland Garros.
He will be placed in the opposite half of the draw to world No 1 Djokovic, who is attempting to win a fourth consecutive Grand Slam.
Not since 2015/16, when Djokovic last accomplished the feat, has any player held all four major titles at the same time.
And Henman reckons the top two players in the world will dominate on the red dirt once again.
“I still think on a clay court Nadal is the favourite,” Henman told the Metro.
“But Djokovic has won three majors in a row and is back.
“I think he got a little bit distracted with stuff going on off the court if you like with the ATP board and chairmanship and all those elements.
“But he seems to have knuckled down and played some great tennis in Rome but, you know, it’s a brave person to bet against Nadal in Roland Garros.
“Could [Stan] Wawrinka? He’s won it before.
“He probably hasn’t got the form right now but I’d be very surprised if the winner comes from outside of Nadal, Djokovic and Thiem.”
However, Henman has warned the leading players not to underestimate world No 3 Federer, who turned 38 later this summer.
“He’s too good a player to write off in any tournament,” Henman added.
“You’ve seen the way he’s played in Rome and Madrid and he’s still probably top-seven or eight on clay.
“He’d need things to go his way but I would never write him off in any tournament.
“But I think on clay, at Roland Garros over five sets, you know given his age it’s going to be the biggest challenge he faces.”