Spanish world number one Nadal and Serbian number two Djokovic - who are also in Paris for the Masters - said earlier in the week that they are reconsidering whether to play at King Abdullah Sports City in Jeddah just before Christmas.
Federer's procession came after Novak Djokovic celebrated his return to the summit of world tennis with an even easier stroll into the last eight after Damir Dzumhur had to retire injured in the second set of their third-round clash.
"I enjoyed it, I feel, in terms of tennis, better than what I thought one week ago, but in the last few days I started to feel a little bit abdominal pain, especially when serving". Maybe I could play today, but the doctor said that if I want to try to win the tournament, the abdominal will break.
It marked the culmination of a dizzying ascent by the Serb, who had slumped to No 22 in the rankings in mid-June, but subsequently won Grand Slams at Wimbledon and the US Open as well as Masters 1000 titles in Cincinnati and Shanghai. For Nadal it means delaying his long-awaited comeback from injury which has kept him sidelined since the US Open. "I had a commitment since one year to play there", Nadal said.
After some speculation about whether or not Roger Federer would actually compete in Paris, he confirmed his participation on Tuesday.
Calling a halt in play, Djokovic went over to the struggling fan while gesturing to the umpire, as Sousa also held up his racket in a signal to stop.
Federer defeated Italy's Fabio Fognini 6-4, 6-3 as the Swiss great moved three wins away from his 100th career title.
In the quarter-finals, Khachanov will meet Germany's fourth seed Alexander Zverev, who took 80 minutes to overpower and finally subdue the scurrying defiance of Argentina's Diego Schwartzman 6-4, 6-2.
After beating Roberto Bautista Agut 7-6 (12-10) 6-4, Dimitrov said: "We've known each other for quite some time and it's the right timing, right now, to have [Agassi] on the team".
Japan's Kei Nishikori continued his bid for a place in the season ending ATP Finals in London a 7-5, 6-4 victory over Adrian Mannarino.