Former world number one Marat Safin believes Rafael Nadal deserves more credit as a tennis player in comparison to Roger Federer because of how hard he had to work.

2017 saw both Nadal and Federer experience a career renaissance as the two superstars split the four Grand Slams between themselves after returning from injury lay-offs last year.

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The duo also won a combined 13 titles in the calendar year and both occupy the top two in the ATP tennis rankings with Nadal reigning at the top spot.

However, it's the Spaniard rather than the Swiss who has impressed Safin more, as he believes Nadal had to truly slog to compete with Federer's all-round natural talent.

"Nadal is more impressive than Federer," Safin told Live Tennis via Metro. "The latter had everything to be successful, while Nadal is a true wrestler and worker."

"He's sweating more and does not get tired. For Federer, it's all easier, though Nadal gives the impression that tennis is a simple thing for him."

Nadal and Federer may be at the top of the mountain right now but Safin has warned them that their long dominance in the rankings could come to a halt as they continue to get older.

"Federer and Nadal have been there for a long time," he added. "If we look at the top-10, nothing has changed since I retired in 2009."

"But that will change. Those in their 30s will soon give their place to young people, who are faster and will improve with time.

"Alexander Zverev has so much potential, but he is still too young. In a year or two he will be a different player. You still see yourself as a junior, but he is no longer a junior, he has to believe in himself and keep working.

"Today's tennis is more physical and fast. Players have less time to think and it's a good thing."