Grigor Dimitrov earlier stunned seventh seed Wawrinka 6-1, 6-2 in the third round. The exit of 2014 Australian Open champion Wawrinka came a year after he won his first and only Masters 1000 title here.
Wawrinka was well off his game, out in 54 minutes with 41 unforced errors and just four winners against ninth seed Dimitrov. The ninth-ranked is now in danger of losing his top 10 ranking place.
World number two Federer missed the chance to reach the last eight in the principality, weighed down by 38 unforced errors against Monfils.
After losing a break for 3-1 in the opening set and standing 5-3 in the second-set tiebreak, 17-time Grand Slam winner Federer could not make much progress against the flashy French opponent who beat him in a Davis Cup rubber last November,
But the Swiss is far from giving up hope only a few days into the spring clay campaign leading to the Roland Garros start on May 24.
“It was a good week for me anyway to come here and practice with the best, play a couple of matches.?? At least gives me some information, if I’m trying to be a bit positive right now.
“I’m going to go back to Switzerland and practice really hard, you know, from whatever the plan’s going to be from tonight on until Istanbul.
“This is good time I should utilise in the right way. I’m looking forward to that.
“I’m playing Istanbul, Madrid. We’ll see what happens with Rome. It was never sure I was going to play that anyway.”
Fourth seed Milos Raonic beat Spain’s Tommy Robredo 6-3, 3-6, 6-3 while sixth seed Tomas Berdych defeated Roberto Bautista Agut 7-6 (9/7), 6-4.