The French Open is knocking on the door, and Novak Djokovic is limbering up for the Grand Slam at the Belgrade Open.

The world no. 1 won his opening match 6-2, 7-6 (7/4) versus Germany’s Mats Moraing on Tuesday. But the victory didn’t come easy. In the contest that lasted one hour and 40 minutes, Moraing managed to break Djokovic’s serve twice in the second set, taking the match to the tie-break.

“I was twice a break up (sic) in the second set so I maybe could have finished out the job earlier, but credit to him for fighting, for playing really well, for playing very courageous, very bold tennis. I felt pretty nervous from the start. I had an opponent that does not have much experience, but he played like he spent 10 years playing at a top level. The second set was exhausting, I'm mentally spent. I have a day to recover and move on to the next match,” Djokovic, who next plays Argentina's Federico Coria, told AFP after the match.

Djokovic has been looking to hit the purple patch since losing the Italian Open final to Rafael Nadal. He has 18 Grand Slams to his name.

Nadal has 20 Grand Slam titles, and 13 of them are French Open crowns. The Spaniard will look to win his 14th Roland Garros title after the Italian Open victory earlier this month.

Former world no. 2 Alex Corretja believes Nadal will have many challenges on his way to the crown, and Australia’s Dominic Thiem could be the top challenger. “There is Novak of course, there is Tsitsipas. There is Zverev, there is Schwartzman – all who can play well. Of course, we see others like Berrettini, Casper Ruud, these guys can stay with Rafa but I’m just not sure they can over five sets. If Thiem is well, then he is probably the one that Rafa can suffer the most against on clay because of the styles of games,” Corretja told EuroSport.