Ex-champion Rafael Nadal slumped to a shock first-round exit as world number 135 Steve Darcis pulled off one of the biggest upsets in Wimbledon history.
The Belgian was inspired as he recorded a 7-6 (7-4) 7-6 (10-8) 6-4 win on Court One to condemn the fifth seed to a second successive early exit at SW19.
It was his first competitive grass tie since Lukas Rosol beat him last year.
However, reigning champion Roger Federer eased into the next round with a 6-3 6-2 6-0 win over Victor Hanescu.
Spaniard Nadal, crowned French Open champion for a record eighth time earlier this month, showed signs he was struggling to adjust from the clay to grass in the opening set and Darcis took advantage.
And the Belgian showed no signs of nerves as he broke in the first game of the third set before serving out for a famous win which condemned Nadal to his first ever first-round defeat at a Grand Slam.
He became the first reigning Roland Garros champion to lose in the first round of Wimbledon since Gustavo Kuerten in 1997.
Nadal refused to use the knee problems which kept him sidelined for a lengthy period after last year's Wimbledon exit as an excuse for the defeat.
He said: "There were not a lot of good things for me. I tried my best all the time I had my chances but I did not make it.
"It is difficult to adapt your game on grass. Now is not the right time to talk about my knee. The only thing I can do is congratulate my opponent. It is not a tragedy, I lost, it is sport.
"I'm confident that I will recover and be ready for the next tournaments. I have played far more than I expected after my injury and I will try my best for the next couple of years."
Darcis told BBC Sport: "Rafa did not play his best match here, it is hard when it is your first match on grass.
"I just wanted to play my own game, coming to the net and not playing far from the baseline. I did not target his knee, tennis is hard enough when you are thinking about yourself.
"I have always played well on grass, perhaps not here where I have had some tough draws, but I am really happy. I do not know what to say."
Meanwhile, Federer, the world number three and third seed, needed just 69 minutes to beat Romanian world number 48 Hanescu, hitting a mere six unforced errors in total.
The 31-year-old won his first Wimbledon title 10 years ago and is now looking for title number eight after beating Andy Murray last year.
The former world number one, hit 32 winners and broke his opponent six times in total.
"I've won Wimbledon a few times now but it still feels special," he said.
"I still enjoy myself in the first round and it's a pleasure playing on centre court again this year.
"I thought it was a good first round for me. I'm very happy. Conditions are pretty cold but it releases some of the pressure now."
He will play Ukrainian world number 116 Sergiy Stakhovsky in the second round.
Elsewhere, 14th seed Janko Tipsarevic is out after he was beaten 6-3 6-4 7-6 (7-5) by fellow Serb Viktor Troicki.
Defeat means the world number 14 has lost in the first round in three of the last four years. Troicki will now face Russian world number 77 Andrey Kuznetsov who beat Spaniard Albert Montanes.
Fabio Fognini was the first seed to fall after he was beaten 7-6 5-7 3-6 2-6 by Jurgen Melzer.
The number 30 seed had 12 break points against the world number 36 but could only convert four, a statistic that eventually proved his undoing.
Nadal's conqueror last year, Rosol, is also out after he was beaten in five sets by Julian Reister.
The world number 35 went down 6-3 4-6 7-6 (7-5) 6-7 (7-4) 6-4 to Reister, ranked 121 in the world.
Number 10 seed Marin Cilic came through a potentially tricky test as he beat the 2006 semi-finalist Marcos Baghdatis 6-3 6-4 6-4.