"For a moment at the end of the first set, I thought that I had to retire (from the match)," Nadal said. "I didn't know what's going on. After that, the pain goes a little bit down and finally I was ready to play."
And del Potro, who played great tennis en route to a routine second-set victory, left the court to have his left hip treated after he took a spill in the third set, which Nadal snuck out via another tiebreak.
In the fourth set, Nadal finally got his first service break of the day with a massive forehand winner, as he grabbed a 3-2 lead in the stanza. The Spaniard consolidated the break with a big hold and held on from there.
Nadal is now 6-3 lifetime against the former world No. 4 and former U.S. Open champion del Potro, who missed Wimbledon last year while recovering from wrist surgery.
The fierce Nadal captured the French Open title earlier this month and is also the reigning Wimbledon and U.S. Open champ. He hasn't lost at the All England Club since losing to Federer in the 2007 final. Nadal beat Federer in their classic final here in 2008, skipped the 2009 edition of the tournament due to injury, and titled here again last year.
The 25-year-old Nadal is now 33-4 for his career at the All England Club.
The 10-time major champion Nadal will face American Mardy Fish in the round of eight. The Spaniard is 5-0 lifetime against the American, including an opening-round Wimbledon victory four years ago.
The second-seeded Djokovic improved to an amazing 45-1 this year with a 6-3, 6-3, 6-3 pasting of 19th-seeded French lefthander Michael Llodra on Court 1. The super Serb advanced in 1 hour, 41 minutes by playing ultra-clean tennis, which featured a mere six unforced errors and only one double fault. Djokovic broke Llodra four times, while the helpless Frenchman was unable to break the two-time Grand Slam champion on Day 7.
Djokovic already owns a whopping seven titles this year. His lone loss came against Federer in the French Open semifinals earlier this month. That setback halted a 43-match winning streak.
The two-time Wimbledon semifinalist Djokovic, who was also last year's U.S. Open runner-up, has never reached a Wimbledon final. He's now a four-time quarterfinalist at the AEC.
Up next for the Serbian slugger will be upstart Aussie Bernard Tomic in Wednesday's quarterfinals.
The third-seeded former world No. 1 Federer came back to best 18th-seeded Russian Mikhail Youzhny 6-7 (5-7), 6-3, 6-3, 6-3 on Court 1. The Swiss great needed just over three hours to advance, which he did with the help of 14 aces among 54 winners and six service breaks. Youzhny settled for only one break on Day 7.
Federer is now 11-0 lifetime against Youzhny, including 3-0 at the majors, and will appear in a remarkable 29th straight Grand Slam quarterfinal.
"I forgot completely (the 29th quarterfinal) was on the line to be quite honest, especially once you're in the heat of the moment, of the battle," Federer said. "I thought I played a good match overall."
The 16-time Grand Slam champion Federer has won six of the last eight Wimbledon titles and is now 59-6 lifetime at the world's most prestigious tennis event. His victory on Monday marked his 100th on grass.
Federer hasn't captured a major title since last year's Aussie Open. He lost to Nadal in the finale at Roland Garros earlier this month.
The 29-year-old Federer will face flashy Frenchman Jo-Wilfried Tsonga on Wednesday.
A fourth-seeded Murray, meanwhile, handled 17th-seeded Frenchman Richard Gasquet 7-6 (7-3), 6-3, 6-2 on Centre Court. The Brit smacked 14 aces and was not broken by the nifty Gasquet.
Murray and Gasquet played in front of the Duke and Duchess of Cambridge -- Prince William and his new wife Kate -- on Monday.
"If I'd known they were coming, I would have shaved," Murray said. "I was thinking to myself as I came off I was sweaty and very hairy. I said to them, 'I'm sorry, I'm a bit sweaty.' But it was really nice."
Murray, who has now reached the Wimbledon quarters four years running, is a three-time Grand Slam runner-up, including this year's Aussie Open. The Dunblane, Scotland native reached the Wimbledon semifinals the last two years and is trying to give Britain its first male Wimbledon champ since Fred Perry in 1936.
Up next for Murray will be Spanish lefthander Feliciano Lopez.
A 10th-seeded Fish straight-setted last year's Wimbledon runner-up, Tomas Berdych, 7-6 (7-5), 6-4, 6-4. Fish is the last American standing here, male or female, after closing Berdych out with an overhead smash into a wide-open court after 2 hours, 22 minutes on Court 2.
The Minnesota native Fish swatted 23 aces and was not broken on Day 7.
Berdych, seeded sixth this year, succumbed to Nadal in last year's Wimbledon finale.
The 29-year-old Fish will now appear in his first-ever Wimbledon quarterfinal and his third career Grand Slam quarterfinal. His last round-of-eight appearance at a major came three years ago in New York.
A 12fth-seeded Tsonga reached his second straight Wimbledon quarterfinal by taking out seventh-seeded Spaniard David Ferrer 6-3, 6-4, 7-6 (7-1). Tsonga popped 12 aces and was not broken in the straight-set triumph.
The 26-year-old Tsonga is a former Aussie Open runner-up.
The promising Tomic punched his ticket to his first-ever Grand Slam quarterfinal by beating Belgian veteran Xavier Malisse 6-1, 7-5, 6-4. Tomic held his quality serve throughout on Court 18.
Malisse was a semifinalist here back in 2002.
The 18-year-old qualifier Tomic is the youngest men's quarterfinalist at Wimbledon since former German great Boris Becker back in 1986.
"What a feeling and what a tournament it's been for me," Tomic said. "I'm at a position now where I've never been happier. I'm looking forward to playing on Wednesday."
Tomic is looking forward to playing Djokovic.
"He's a cool guy. One of the nicest guys out there on the tour," Tomic said. "To play against a guy like him that's (No.) 2 in the world, it doesn't get really better than that."
Lopez continued his fine play at the All England Club with a grueling come- from-behind victory over fellow non-seed Lukasz Kubot, 3-6, 6-7 (5-7), 7-6 (9-7), 7-5, 7-5. Lopez needed 4 hours, 18 minutes to complete his Day-7 comeback. The slugfest featured 55 aces, including 28 from the winner, and only seven breaks of serve, including four for the Spaniard. Both players fired 80 winners in the marathon.
All four men's quarterfinals will be held on Wednesday, as Tuesday will be an off day for the men in singles.