PARIS -- It is 17 months since Stan Wawrinka walked into the news conference room at the Australian Open, grimacing in pain after his first attempt at coming back to the Tour after knee surgery.

There were times when he was not sure if he would ever to be able to play at his best level again, the scars of two operations in 2017 to his left knee still raw, physically and emotionally.

But at Roland Garros on Sunday, in five hours and nine minutes of epic, brilliant, emotional tennis against No.6-seeded Stefanos Tsitsipas of Greece on Court Suzanne Lenglen, all the heartache and all the doubts were finally washed away.

Wawrinka is back. And boy, was it a good feeling.

"I never experienced that kind of atmosphere here in Paris on Lenglen," Wawrinka said, exhausted but elated after reaching his first grand slam quarterfinal since he reached the French Open final two years ago. "Always had a lot of support, but I think today was really special, and the match was amazing, five hours' match, five sets. The crowd stayed all the match. This kind of atmosphere, for sure, I enjoyed. I enjoy a lot. That's the reason why I still play tennis, and that's the reason why I'm practicing every day to try to win big matches like that. So for sure, when I'm on the court I try to enjoy and remember also everything I have done to be here."

On a brutally hot day, when temperatures hit 90 degrees fahrenheit and made even watching at courtside uncomfortable, Wawrinka -- seeded 24th -- and Tsitsipas put on an exhibition of ballstriking. No one hits a backhand like Wawrinka, but with Tsitsipas showing off his own stunning one-hander, it was an aesthetic delight.

The five-set, fourth-round match between Stan Wawrinka, left, and Stefanos Tsitsipas was the fourth longest in French Open history, clocking in at 5 hours, 9 minutes. Martin Bureau/AFP/Getty Images

With his long hair kept in place by a headband, there are strong echoes of Bjorn Borg in Tsitsipas, his strut also reminiscent of the Swede who won the title here six times. His shirt was covered in clay, the aftereffects of trying to fend of the Wawrinka passing shots. Aggressive throughout, the Greek created chance after chance, only for Wawrinka to close the door, time and time again. Tsitsipas had a set point in the opener but took just five of his 27 break point chances as Wawrinka shut the door and finally, broke his spirit.

When the match was over, he was reduced to tears.

"I feel exhausted," Tsitsipas said. "Never experienced something like this in my life. I feel very disappointed at the end. Long time since I cried after a match, so emotionally [it] wasn't easy to handle. I will try to learn from it as much as I can."

The conclusion to the match was sheer drama.

Wawrinka floated a sliced backhand that curled into the court and landed on the line. Tsitsipas, who had left the volley, circled what he thought was the mark but the umpire, down from his chair, ruled it in, a decision accepted by Tsitsipas as Wawrinka stood, arms above his head in sheer joy.

"When I hit it, I saw it was on the line. I was just hoping that the umpire would confirm it," he said. "So I was half a second not sure, but when I saw he was going to say it's in ... it was amazing battle today. Today we saw [that] only one centimeter can change the winner."

Only a handful of seats were left untaken throughout, but if anyone enjoyed the match more than the crowd of almost 10,000, then surely it was Roger Federer, whose work was done and dusted a few hours before Wawrinka left the court.

At 34, and having played for three straight days after his previous match was held overnight, Wawrinka faces a battle to be ready to face his old friend, who cruised past Leonardo Mayer 6-2, 6-3, 6-3 and has yet to drop a set. It is four years since Wawrinka beat Federer in straight sets in the quarterfinals here, the year Wawrinka won the title. It was a dominant win both men remember well, one in which Federer says he was "crushed."

"I'm happy he say that for once in his life," Wawrinka said, smiling. "I think I crushed the tournament that year, so I'm happy with that memories. But ... it didn't happen many times in my life against him. Normally it's always the opposite. Not much else to say. We all are happy that he's back on clay court after few years. We'll see for me in two days."

Taking questions from French-speaking journalists, Wawrinka admitted that recovering in time to play Federer will not be easy.

"Obviously it leaves a trace," he said. "It's heavy. We'll see how I recover, because it takes its toll, and I'll have to recover before Tuesday."

Nothing is impossible, not for Federer, who at 37 continues to defy all logic about what's possible years after players even a decade ago would hang up their rackets. In his first Roland Garros since that beating by Wawrinka in 2015, he has sauntered through the draw.

With his victory Sunday, Stan Wawrinka in the quarterfinals of a Grand Slam for the first time in two years. EPA/Julien de Rosa

Now things get tougher. If Wawrinka can recover physically, then Federer will have to be close to his best, even if he leads their head-to-head record 22-3.

"On clay, this is when it's been most tough for me against him," Federer said. "If I think back [to 2014] Monaco finals, French Open here in '15 and then also he beat me in Monaco another time. So on clay it's been definitely more dangerous than on any other surface for me against him. I'm just happy for the guy that he's back after his knee problems. They were severe, and that's why I think he's really happy he got sort of a second life on tour, because I think for a while there, he wasn't sure if he was ever going to come back again. It's nice to see him pain-free and playing well. I hope he's not at the level of '15, but we'll find out, because there he was crushing the ball. It was unbelievable."

Whether Wawrinka can recover and do it again is another question, but on Sunday, it was just like old times.