After serving an ace down the middle on match point, Darcis conceded he was as surprised as everyone.
"Nobody was expecting me to win," he said. "So I had to play a good match, relax, and enjoy the game. That's what I did."
There were no surprises for the other big names: Defending champion Roger Federer, bidding for a record eighth Wimbledon title, and second-seeded Andy Murray both won in straight sets on Centre Court.
Nadal was sidelined for seven months with a left knee injury after losing in the second round of Wimbledon last year. He seemed to be struggling physically Monday. He was unable to turn on the speed or use his legs to spring into his groundstrokes, limping and failing to run for some shots.
Nadal declined to blame any injury and gave full credit to the 29-year-old Darcis, who had never beaten a top-5 player before and has yet to go beyond the third round of any Grand Slam.
"I don't ... talk about my knee this afternoon," Nadal said. "Only thing that can say today is congratulate Steve Darcis. He played a fantastic match. Everything that I will say today about my knee is an excuse, and I don't like to put any excuse when I'm losing a match like I lost today."
Darcis, who had won only one previous match at Wimbledon, played the match of his life Monday, going for his shots and moving Nadal from corner to corner. Darcis amassed a total of 53 winners, compared with 32 for Nadal.
"Of course, Rafa didn't play his best tennis," Darcis said. "I could see it. So I took advantage of it, tried to fight. Maybe he was not in the best shape ever. Maybe he didn't play his best match. But I have to be proud of me, I think."
P Tuesday, 5 a.m.
TV • ESPN