Six-time Grand Slam champion Boris Becker has hit back at Rafael Nadal's critics by insisting that the Spaniard will return to his best.

Following various injuries, the 29-year-old has had a slump in form, which has dropped him down to number eight in the world.

Despite recent results, Becker, who coaches world number one Novak Djokovic, does not believe that Nadal's best tennis is behind him.

"The criticism Rafa got this year was unfair because he put his level so high by winning nine of the past 10 French Opens, and being number one or number two in the world for so long," Becker told Sky Sports News.

"It's obviously a big drop but he hasn't forgotten how to play tennis. He was injured for a while, last year [at the US Open] he didn't play at all so he's only had half a year. It's a question of time for when we see the old Rafa back."

Nadal will take on Fabio Fognini in the third round of the US Open this evening.