"I think of Novak (Djokovic) the way I used to think of Serena (Williams) – he is always the favorite, Rafa (Nadal) is still incredibly consistent" - Todd Martin

Novak Djokovic is "always the favorite" while Rafael Nadal is "still incredibly consistent."

These are the assessments of former World No. 4 and 1999 US Open finalist Todd Martin about the two top contenders for the Wimbledon crown in a tweet by journalist Sasa Ozmo.

Martin expounded on the strengths of Djokovic and Nadal, pointing out how mentally and physically sharp three-time defending champion Djokovic is despite the challenges he has faced this year and how Nadal remains consistent and focused despite playing below par.

"I think of Novak (Djokovic) the way I used to think of Serena (Williams) – he is always the favorite, it's always him. This year has been challenging for him because he hasn't been able to play a lot, but I feel like he's mentally focused and physically in great shape. When that's the case, I still believe he is the best player in the world," said Martin, now the CEO of the International Tennis Hall of Fame.

"On the other hand, Rafa (Nadal) is still incredibly consistent, even when he's playing 5 to 10 percent below his best level. He is still insanely focused, and that's why I think we see more ups and downs from Novak than we do from Rafa," he added.

Martin also emphasized Djokovic's return game, for him the best of all time, which now works more on grass, as it does on clay and hard-courts.

"That's they key – defensive skills were always Novak's strength, and now it's rewarded more on grass, as it is on other surfaces.

"He (Novak Djokovic) has the best return of all time, in my opinion. It's going to stay that way until he retires and it's going to be very difficult to find someone who is going to be able to do it better – one of the reasons is that his return is equally effective on all surfaces," said Martin.

As for Nadal, his physicality remains in question, but he is "playing spectacularly this season," according to Martin.

The two-time Wimbledon champ also boasts of experience, as does his rival, and that makes them the biggest hurdle for anyone eyeing the crown, added the former American tennis player.

"Only Rafa knows how physically ready he is... He probably doesn't quite feel perfect but he is getting there. He is playing spectacularly this season, and he also got the experience of winning here.

"Novak and Rafa are the biggest challenge because they know how to win a title here. They've done it before," he said.

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Novak Djokovic-Rafael Nadal potential Wimbledon final clash still on the horizon

Novak Djokovic and Rafael Nadal hold their respective trophies after their Wimbledon final clash in 2011.

Novak Djokovic and Rafael Nadal fans could be treated to another epic final battle between the two for the Wimbledon crown.

But first, they have to get past their respective opponents in the Last 16 – Djokovic against Jannik Sinner and Nadal against Indian Wells foe Taylor Fritz.

Djokovic and Nadal faced off three times in the grass-court Major, all in the late stages of the tournament. They last met in the semifinals four years ago, with eventual champion Djokovic scraping past Nadal in five sets. They also squared off in the finals in 2011 and the semifinals in 2007, which they split 1-1, with Djokovic retiring in their first Wimbledon meeting.