At 37, and boasting a striking peroxide blond hairstyle that some online critics have likened to a cross between punk rocker Billy Idol and Bart Simpson, the Italian clinched a 6-4, 7-5, 6-7 (1/7), 6-3 win.
He did it the hard way, needing multiple match points to see off Ruud, a three-time Grand Slam runner-up.
"This is why I love and hate this sport at the same time," said Fognini, who is playing the tournament for the 14th time but has yet to make the second week.
"But I was really calm. Even if I was losing this crazy third set, I was thinking, 'OK, I'm two sets to one up. I start again'.
"He's in the top 10 and I'm 37 (years old) so this is a big present for me."
Fognini is a former world number nine but is now down at 94.
He has just six main draw wins all year on the ATP tour and has even drifted down into the second-tier Challenger circuit in an attempt to rediscover some form.
"I still love to play even if the ranking is going down and my age is going up," said Fognini before attempting to mend fences with the tournament.
In 2019, after playing and losing out on Court 14, he was overheard saying: "It's fair to play here? Damn English, really. Damned, really. I wish a bomb would explode on this club. A bomb should explode here."
He later apologised for the remarks.
On Wednesday, in front of a packed if cold Court Two crowd, Fognini confessed to a change of heart.
"It's the best Grand Slam in the world compared to the other three," he told fans.
"I love the competition. I want to enjoy it as much as possible. Maybe this 14th time will be lucky for me."
