Rafael Nadal and Novak Djokovic
Novak Djokovic's biggest win on clay over Rafael Nadal came in the 2015 French Open quarterfinal. Pictured: Spain's Rafael Nadal (L) shakes hands with Serbia's Novak Djokovic at the end of their men's quarter final match of the Roland Garros 2015 French Tennis Open in Paris, June 3, 2015. PATRICK KOVARIK/AFP/Getty Images

World No. 1 Rafael Nadal singled out Novak Djokovic as a player who gave him trouble on his preferred clay surface.

Nadal's dominance on clay continued after a 6-3, 6-4 win over Argentina's Diego Schwartzman on Thursday saw him advance to the quarterfinals of the Madrid Open.

Having already won the Monte-Carlo Masters and the Barcelona Open last month since returning from a hip injury suffered in January, the Spaniard is now looking for his third title of the year as he faces Dominic Thiem next on Friday.

Nadal is yet to be tested since his return to competitive action and with his win over Schwartzman, notably broke the record of most consecutive sets won (50) on any surface after surpassing John McEnroe's record of 49 on the now-discontinued carpet surface.

However, Thiem is viewed as one player who could give him problems with the Austrian prospect even recently claiming the 31-year-old could be beaten on clay, particularly in Madrid and the Italian Open, the latter being where he defeated Nadal last year.

When asked whether Thiem was his most dangerous foe on clay, Nadal agreed, though he stated it was mainly because he would be playing him next. However, he did go on to mention Djokovic as well.

"Right now, for sure, because he's [Thiem] the one I have to face tomorrow," Nadal said, as per Metro. "This is what we have today. After that, things we cannot see in the future. I am not saying just in front of you guys, but I say it because I feel it that way."

"The most dangerous opponent is the one you have to face the next day. After that there are times where there is an opponent that perhaps is tougher, maybe some moments it was Djokovic."

Djokovic has one of the more impressive records on clay against Nadal, defeating the 16-time Grand Slam champion seven times in 21 meetings with his most impressive win by far coming in their 2015 French Open quarterfinal clash.

But as for his toughest opposition on the surface today, given Djokovic's drop in form since the start of last year, Nadal believes it is likely that the player has not reached his full potential yet and it could even turn out to be Thiem.

"Today it's not very clear. I just try to do my part, to walk my path," Nadal added. "I know that there are players with a lot of potential that perhaps still haven't reached that level or that consistency that we've had during so many years. They truly have the potential. One day they will be very dangerous."

"Tomorrow might be one of those days that I'm going to play one of the best players of the world [Thiem]. Of course, he is one of the best players of the world because the ranking speaks for itself. But on this surface, he’s one of the most dangerous players you can face, one of the ones that have a lot of possibilities to succeed," he said. "Tomorrow is a key match in the tournament. It’s one of the matches where I have to be a step ahead. Well, I think I'm prepared for that step, but I have to do it tomorrow."