It was a stress-free match that took 14 minutes short of two hours. Nadal faced only two break points and saved them both, with neither the wind nor his 13th-seeded opponent causing the slightest trouble.
Nada's 19 dropped games in the tournament are the fewest to this point at Roland Garros since 1982, when Guillermo Vilas dropped only 16. Like Nadal, John McEnroe also made it through four rounds while losing only 19 games at the 1984 U.S. Open.
"I feel really comfortable, really at ease," said Nadal, who is tied with Bjorn Borg with a record six French Open championships. "When the tournament is over, I'll tell you if this was my best Roland Garros or not. For the time being, I'm still playing. So far, so good. But we'll see. Things could change."
He trailed 2-1 at the start against Monaco, one of Nadal's best friends on tour, before reeling off the last 17 games in a row.
"I feel very, very sorry for him," the No. 2-seeded Nadal said after improving to 49-1 in the French Open over his career.
The only loss came in the fourth round in 2009 against Sweden's Robin Soderling.
He broke Monaco eight times.
He made only 13 unforced errors. That raised his total for the tournament to 64 -- or 13 fewer than No. 1-seeded Novak Djokovic made in his fourth-round match alone.
"When you reach quarterfinals with my results, that's always something positive, that's true. ... I have won quite easily. My results are good," Nadal said. "But I'm not going to go through immediately [to] the semifinals."
Almagro hasn't lost a set at this year's French Open.
Nadal, Almagro said, is "the man on clay, and we are trying to [get] close to him. We are working hard to do our best."
Fifth-seeded Jo-Wilfried Tsonga of France also advanced, closing out a five-set victory over Stanislas Wawrinka by winning the last two games of a match that was suspended by darkness the night before.
Almagro's previous two trips to the round of eight at Roland Garros, in 2008 and 2010, both ended with losses to six-time champion Nadal, part of an 0-7 career mark against his countryman.
Tsonga, trying to become the first Frenchman to win any Grand Slam title since Yannick Noah's championship at Roland Garros in 1983, defeated Wawrinka 6-4, 7-6 (6), 3-6, 3-6, 6-4.
Tsonga led 4-2 in the fifth when play resumed but gave up a quick break. He then broke back at 5-4 to close out the win and advance to his first French Open quarterfinal, where he'll play Djokovic.
"To a certain extent, it was a bit of a nightmare until the moment I hit my first ball, because before that, I had a thousand questions in my head," Tsonga said. "I really wanted to win that match, and it was very difficult."
Tsonga, the 2008 Australian Open runner-up to Djokovic, is 5-5 lifetime against the Serb, who is trying to become the first man since 1969 to win four straight Grand Slam tournaments.
The Frenchman will clearly have the fans on his side, though he's well aware that isn't always enough against a player like Djokovic.
"It's going to be a very difficult match," Tsonga said. "But obviously I'll fight like a lion and we'll see the result. I'll do everything I can to make it a difficult match for him."
The five sets lasted 4 hours, 6 minutes, most of it played in the dimming light the night before. Still, Tsonga said he felt as if the hardest work came when they restarted on the second straight cloudy, blustery day at Roland Garros.
"I've spent more energy in four games today than in five sets yesterday," he said.
Ferrer wiped out No. 20 Marcel Granollers 6-3, 6-2, 6-0. Ferrer has dropped only 25 games en route to the quarterfinals.
"It's true that in the past four matches, I felt really comfortable. I felt really at ease," Ferrer said. "I think I have to continue this way."
Del Potro finished off a 7-6 (6), 1-6, 6-3, 7-5 victory over Tomas Berdych in a match stopped the night before because of darkness after three sets.
Del Potro broke No. 7 Berdych at love in the final game.
Del Potro is 2-11 overall against Federer, including a loss in the 2009 semifinals at Roland Garros, and was asked the key to winning this time.
"Play an unbelievable match. Try to take my opportunities. Serve 100 percent. Trying to play winners with my forehand, with my backhand, and [force] him to raise his game," del Potro explained, then went on a little longer, capping the list with "unbelievable shots."
Berdych was a French Open semifinalist in 2010 then was the runner-up at Wimbledon later that year.
Murray complained his back was hurting him in the early rounds last week. He was clutching his back during the first set Monday in the cold and wind at Roland Garros.
But as the match went on against Gasquet, seeded 17th, the fourth-seeded Murray showed no signs of injury.
The match ended oddly, with fans pulling for the Frenchman, Gasquet. They delayed the start of the last game by doing an extended wave, then cheering when Murray served into the net and mockingly saying "shhhh" when the umpire tried to quiet them.
"I wouldn't say it got too much," Murray said. "I mean, yeah, it's almost like playing a sort of a football match. And I like football. I enjoyed myself on the court today. It's the most fun I've had on the court in a while, so I wasn't shying away from the fact that the crowd wanted me to lose."
Murray's win means this year's French Open is the first Grand Slam tournament since the 1984 French Open with all top six seeded men in the quarterfinals.
Information from The Associated Press was used in this report.