PARIS — As recently as two days ago, it was widely believed that the hardest task in tennis was to beat Rafael Nadal at Roland Garros.

But after a stunning come-from-behind win over Stefanos Tsitsipas in the French Open final on Sunday, following a defeat of Nadal in the semifinal, perhaps the mantra should be changed: The hardest task in tennis is to beat Novak Djokovic in a Grand Slam event.

Tsitsipas, the leader of the so-called Next Gen, thought he had him beaten, taking the first two sets in the final on Sunday under perfect, sunny skies. Djokovic, who had needed more than four hours to wear down Nadal in an epic semifinal, looked spent.

We should have known better. For the umpteenth time in his career, Djokovic was at his most dangerous when he looked done — pulling himself back yet again to clinch a 6-7, 2-6, 6-3, 6-2, 6-4 victory.

The 34-year-old now owns 19 Grand Slam titles, one behind the record of 20 held by Nadal and Roger Federer. He is the first man in the Open era to win all four Grand Slams at least twice. And whisper it, but he is halfway to the coveted achievement of winning all four slams in a calendar year, something only Rod Laver has managed on the men’s side in the Open era.

“The level of tennis in the last 48 hours was not easy to do,” Djokovic said Sunday. “I think beating two great champions [Nadal and Tsitsipas] was physically, mentally very tough. Once again, it’s a dream come true. I’m not going to stop here, I’m going to keep going.”

Tsitsipas had also needed five sets to get past Alexander Zverev in the semis, holding his nerve when the German fought back from two sets down to force a decider.

In his first Grand Slam final, Tsitsipas came out strong and matched Djokovic in the first set, saving a set point at 6-5 and then another in the tiebreak as he moved ahead. In the second, Tsitsipas stepped it up again, and by the time he had doubled his lead, the world No. 1 looked shattered.

Djokovic took a bathroom break after the second set, and when he returned, things changed.

“He came back, to me, like a different player, suddenly,” Tsitsipas said. “He played really well, he gave me no space. I felt physically, everything on the court [for him] felt much better than before. I felt he could read my game suddenly. Good for him; he did well to get there.”

Even though Tsitsipas continued to battle, the real Djokovic had returned. The Serb lifted his game several levels, with his drop shot and touch around the net particularly effective.

“After that [break], I felt like I got into his head,” Djokovic said. “I feel like I started swinging through the ball better. The momentum was on my side — it shifted. There was no looking back from that moment.”

Federer could probably sympathize with Tsitsipas, having experienced first-hand Djokovic’s resilience on more than one occasion. In five-set clashes at the US Open in 2010 and 2011, Djokovic saved two match points each time in the deciding set before defeating Federer. Then, perhaps most famously, Federer was unable to convert back-to-back match points in their Wimbledon final meeting in 2019.

When Djokovic’s back is against the wall, when he looks like he’s exhausted, he is like Muhammad Ali on the ropes, a moment away from bouncing back and turning things around in his favor. His two comebacks from two sets down at this French Open made him the first player in the Open Era (since 1968) to make multiple 2-0 comebacks en route to winning a major singles title, per ESPN’s Stats & Information Group. Djokovic is also the third player in men’s tennis history with multiple 2-0 comebacks along the way to winning a major title. The others are Henri Cochet at 1927 Wimbledon (three comebacks from two sets down) and Ted Schroeder at 1949 Wimbledon, per SIG.

Tsitsipas summed up what it is like to face him, and what it takes to win a Grand Slam title.

“What I learned today is that no matter what, in order for the match to be finished, you have to win three sets and not two,” he said. “Two sets doesn’t really mean anything. It’s still one away from winning the entire match.

“It’s all about endurance; if you can keep up with the endurance, keep your level there for longer periods of time, then of course that’s what is needed in a Grand Slam. I played two good sets. I wouldn’t call them incredible. I just played really well. It wasn’t enough. It wasn’t enough. That’s a Grand Slam for you. It’s the way it is.”

Speaking on Eurosport, three-time Roland Garros champion Mats Wilander said Djokovic’s ability to rally from difficult situations was unparalleled.

“I’m not sure what he tells himself when he is two sets to love down — is he able to completely forget the score or is it that he has spent two hours on the court with someone and now knows exactly what he needs to do?” the Swede said.

“He pulls off the right shots at the right time, he looks fresher and fresher as the match is going on. He’s so clever and he’s so calm these days. He looks unbeatable in the third, fourth and fifth.”

Tsitsipas should take solace in reaching his first Grand Slam final and the fact that he will rise to a career-high No. 4 in the rankings when the list is updated Monday.

For Djokovic, though, the quest for ultimate greatness continues. He will be the favorite to win at Wimbledon, which begins in a fortnight, with a chance to equal Federer and Nadal at 20 Grand Slams.

Should he manage that, he will then go to the US Open looking to break the record and complete the calendar-year Slam. Few would bet against him.

“I think he has the ability to win the Grand Slam for this year,” Marian Vajda, Djokovic’s coach, said Sunday. “I’m pretty sure.”