“He did a nice job of taking care of his half volleys,” Federer said. “That’s maybe what won him the match tonight, I’m not sure.”

Perhaps pride kept Federer from equivocating. The difference in the match was Tsitsipas’s youthful energy, which he oozed with his every Tigger-like step. Federer simply couldn’t match it from the first point to the last, a backhand that he netted.

Federer wasted no time after the match completing his media responsibilities, leaving the clear impression that he couldn’t put this loss in his rearview mirror fast enough. His eyes grew glassy as his news conference wore on, as if his feelings about the loss had traveled from his mouth to his marrow.

“I have massive regrets, you know, tonight,” said Federer, who also announced he intended to play some events on clay this spring after skipping clay-court season the past two years. “I might not look the part, but I am. I felt like I have to win the second set. I don’t care how I do it, but I have to do it.”

Tsitsipas, who trains at the academy of Patrick Mouratoglou, whose best-known pupil is the 23-time major winner Serena Williams, was proud of the mental toughness he showed in fending off a dozen break points in all.

“I could have cracked at any moment,” he said, “but I didn’t because I really wanted it bad.”

Tsitsipas said he believed from the first point that he could pull off the upset and described the victory as “a beginning of something really big.”

Tsitsipas broke onto the scene last year by reaching the final of the Rogers Cup in Toronto in August, defeating four top-10 players, including Novak Djokovic, on the way.