Roger Federer reached the last 16 of the French Open but needed all of his fighting qualities to scrap his way past Germany's Dominik Koepfer on an empty and eerie Court Philippe Chatrier in Saturday's night match.

The 39-year-old 20-time Grand Slam champion was ragged at times and lacked his usual zip against the tenacious Koepfer but the eighth seed slogged his way to a 7-6(5) 6-7(3) 7-6(4) 7-5 victory with the time approaching 1am local time.

It was Federer's first taste of a Grand Slam match played without fans, courtesy of Paris's 9pm COVID-19 curfew, and it was not an experience he seemed to enjoy as he was given a torrid time by the 27-year-old left-hander.

Yet the fact Federer prevailed in such surreal circumstances, having hardly played in the last 17 months because of a knee injury, illustrated the passion that still burns within him for the game.

Former world number one Andy Murray, watching at home, summed it up nicely on Twitter.

"I'm not bothered by the outcome of this match at all. Just seeing Federer at 39 off the back of 2 knee surgeries playing to an empty stadium at 12.30am getting fired up is inspirational to me. Do what you love," the Scot wrote.

Federer had looked superb in his opening two matches here, but was rattled by the heavy hitting of the world number 59 and struggled for rhythm, racking up 63 unforced errors.

But he dug deep to win the night's opening tiebreak, helped by a Koepfer double fault at a crucial moment.

Federer piled on the pressure at the start of the second set and secured an early break. But Koepfer broke back and after breaks were traded again, the set reached a tiebreak.

Koepfer moved 6-2 ahead as mistakes flowed off Federer's racket and he levelled the match thanks to another unforced error from the Swiss who was playing only his sixth match in 17 months following knee surgery.

Federer, the 2009 champion, was wobbling when he dropped serve early in the third but battled back from 2-4 down and after missing a set point in the 12th game he reeled off the last three points of the night's third tiebreak.

Sensing his chance of taking the biggest scalp of his career was slipping away, Koepfer's temper boiled over when his service was broken at 1-1 in the fourth and he was docked a point for spitting on the court after a close call went against him.

The former American college player was not done though and Federer could not locate the extra gear he usually engages to see off troublesome opponents, dropping serve in the next game.

Federer showed amazing reflexes to return a smash when Koepfer served at 5-5 and he pounced for a crucial break as Koepfer struck a forehand over the baseline.

The Swiss then held serve to claim a gruelling victory, albeit an ugly one by his usual standards, and set up a clash with Italian Matteo Berrettini.

"It was very important for me, I certainly hadn't practised for three hours 35 minutes," Federer told reporters. "I certainly didn't expect to win three matches here.

"Playing without fans I knew was going to hit me at some point. It maybe benefits players like myself who are very focussed when they go to practice. It wasn't easy tonight, was unique in many ways, but I'm happy I found a way.

"I was playing for all those watching at home on TV."