World number one Nadal, champion here in 2008 before rolling on to win the Wimbledon title, needed treatment on his backside during an awkward third-round match against Uzbekistan's Denis Istomin before scraping through in three sets.
Serbia's second seed Novak Djokovic joined Roddick out the exit door, losing to Belgium's Xavier Malisse 6-3, 4-6, 6-2 while defending champion Andy Murray was tied at 3-3 in the deciding set with American Mardy Fish when bad light stopped play.
Fifth seed Marin Cilic also tumbled on a bad day for the top seeds, the Croat beaten 7-6, 6-2 by France's Michael Llodra.
Nadal did survive, just and insisted he would continue in the tournament despite his injury scare.
"I just had a pain behind the leg and wanted the physio to check that it wasn't dangerous to carry on," the 24-year-old Spaniard said after a 7-6, 4-6, 6-4 win in a cool, swirling wind.
"I felt something and I will have some more tests. I will be careful but I'd love to play tomorrow," he added.
Nadal, who won a fifth French Open title on Sunday, gets to play at least one more singles here to help with the tricky transition between slow bouncy clay and skiddy London grass.
RODDICK OUTPLAYED
Not so Roddick. He suddenly has more time on his hands than expected after the four-times Queen's champion and three-times Wimbledon runner-up was outplayed by Sela.
The American played solidly enough but Sela, ranked 63 in the world, had an answer to everything as he recorded a 6-4, 7-6 victory in which he offered up just one break point opportunity.
Roddick, 27, had his one chance on Sela's opening service game but failed to take it and then dropped serve to trail 2-1 and played second fiddle there on in.
"I'll probably get more time in on the track or in the gym, maybe figure out if there's some shows or a concert to kind of get away and take my mind off the fact that I'm not playing here," Roddick told reporters.
Despite playing just two matches here, Roddick said he had no plans to take a late entry at Eastbourne next week.
"I'm not panicking. I'm not real worried about it. I wish I could play some more matches, but it will be fine," he said.
"I'd be lying if I said I'm looking forward to practising the next two days as opposed to playing. That part is gonna suck," he added.
Sela will face Rainer Schuettler after the German moved into the last eight thanks to Frenchman Richard Gasquet pulling out with a back injury before their third round match.
Nadal pinched a rain-interrupted first set against Istomin on a tiebreak but never looked completely in control of his game, making more errors than usual, and his burly opponent broke early in the second to level the match.
After taking an injury time-out Nadal appeared none the worse for wear and stayed ahead on serve. He recovered from 15-40 at 3-3 then broke Istomin in the next game to serve for the match before suffering a lapse.
At 40-30 he made a hash of a routine smash and Istomin seized his chance to break back.
Nadal sealed victory against serve in the next game and will face fellow Spaniard Feliciano Lopez for a semi-final berth.
"The positive thing is that I was mentally there in the third, " Nadal said. "I finished the match playing better."
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