The tennis icon was set to play with great rival Rafael Nadal (36) in the evening doubles slot on the opening day of the Laver Cup in what will be his last professional match.
Federer, a 20-times Grand Slam champion and regarded as one of the greatest players of all time, announced last week that a persistent knee injury meant he was ending his 24-year career.
Tickets for his match with Europe team mate Nadal against American duo Frances Tiafoe and Jack Sock were like gold dust.
But some fans got lucky.
Many walking into the Thames-side venue where eight-times Wimbledon champion Federer won the ATP Finals title in 2010 and 2011 were draped in Swiss red and white and had handkerchiefs at the ready for what promised to be an emotional evening.
"He is one of the best ambassadors for Switzerland so I'm super happy to be here," 31-year-old Sonya Carvallares from Zurich told Reuters.
"He is more than just a tennis player he is an icon."
Federer has not played a competitive match since last year's Wimbledon and Carvallares said the news that he was ending his golden career was sad, but expected.
"We saw it coming right, he is not eternal. On a personal level I'm very happy we got the tickets for today especially now. But it's mixed feelings in a way.
"In terms of number he's not the greatest, but in our hearts he is the greatest. I think it's going to be emotional, I might have a few tears in the eyes."
GLOBAL APPEAL
While many fans came from Europe, Federer's unique appeal spans the globe.
Miraaj el-Haque, dressed in a sweatshirt emblazoned with Federer's caricature and the slogan "Greatest of All Time" travelled all the way from Bangladesh.
"The entire first chapter of my life from when I was five to now I'm 23....I've known nothing but him in the sports world," he said outside the O2 Arena.
Asked what makes Federer so special, he said: "I think how relatable he is, and how much of a role model he is and how he makes a fan feel when he plays."
"I think his connection with the fans is something I don't think any other player will have for a while.
"The way he makes you feel, the way he plays the game, the way he conducts himself off the court, I think all those factors make him the greatest athlete in any sport."
Sadra Mahoob travelled from Tehran to watch Federer for one last time.
"Roger is a superstar in Iran, everyone knows about him and had the dream to watch him in a live match," he said.
Swiss student Aurille von Ruht said the finale, with Nadal, was the perfect way for Federer to go out.
"Rafa and Roger are two beautiful players and two beautiful people, so I'm happy Roger gets to finish like this."
