Roger Federer's ex-coach Paul Annacone cannot see anyone surpass the 20-time Grand Slam winner or Rafael Nadal in the 2018 season.

Both players returned from long injury layoffs to enjoy career resurgences last year as they won a combined 13 titles and split the four Grand Slams between themselves, with Nadal ending the year as the world number one for the first time since 2013.

While Federer came up short in his chase to cap a remarkable year as the top-ranked player, his success has continued in 2018 as he was able to retain the Australian Open last month.

The Swiss legend followed it up by accepting a wild card to enter the Rotterdam Open and went on to win his second title of the year on Sunday (18 February) and overtook the Spaniard to become the oldest world number one in the process at 36 years of age.

With Nadal expected to dominate the clay-court season later this year as the duo continue their remarkable rivalry, Annacone believes the two long-time rivals will continue to be ahead of everyone else on the Tour.

"If you look at the way Roger and Rafael have dominated the sport the last 14 months it is hard to see either of them being surpassed, especially the way Nadal dominates the clay," Annacone told the Tennis Channel via Express.

"Obviously there is the variable of injuries and the variable of how often Federer plays. Roger wins the way he does, because he's so selective with his scheduling.

"Rafa is going to play a lot more and he's going to absolutely nudge up those clay points if he's healthy."

As of now, however, Nadal is not fully fit, having withdrawn from his Australian Open quarter-final match-up with Marin Cilic due to a hip problem.

The 16-time Grand Slam winner, though, is expected to return to action at the Mexican Open in Acapulco which takes place from 26 February.

Roger Federer
Roger Federer and Rafael Nadal have won a combined 15 titles since the start of 2017 Getty