The 20-time Grand Slam champion was shocked by a man 17 years his junior in the last 16 of the Australian Open 6-7 (11-13), 7-6 (7/3), 7-5, 7-6 (7/5) on Rod Laver Arena.
But Federer, the defending champion, played down suggestions it was the beginning of the end for a man who has dominated the sport for so long.
"I've heard that story the last 10 years. From that standpoint, nothing new there," he said.
And in a signal that he is hungry for more, the 37-year-old said he planned to play on clay again this year for the first time since 2015.
"It's a bit of a desire. I'm in a phase where I want to have fun and I've missed not doing it," he told reporters of the French Open.
He missed Roland Garros in 2016 with an injury and skipped the clay season the last two years.
"I don't feel it is necessary to have a big break again," he added.
The long-haired Tsitsipas, seeded 14, has been touted as a future Grand Slam champion and Federer had nothing but praise, saying he saw similarities to himself.
"He has a one-handed backhand and I used to have long hair, too," said the world number three.
"Yeah, so maybe a little bit, sure. He has more of a continental grip than players nowadays. That's a bit more my way than, let's say, Rafa's way.
"I see him definitely being high up in the game for a long time. That was a good night for him tonight."
Federer conceded he had "massive regrets" about losing and said he didn't win because he struggled to convert his 12 break points, making none of them count.
"I have massive regrets, you know, tonight. I might not look the part, but I am. I felt like I have to win the second set," he said.
"I don't care how I do it, but I have to do it. Cost me the game tonight.
"There is always multiple factors that play into a match like this," he added.
"But it definitely didn't go the way I was hoping on the break points. I also didn't break him at the Hopman Cup, so clearly something is wrong how I return him, what I'm trying to do."AFP