MELBOURNE, Australia >> Learner Tien, a 19-year-old qualifier from California, became the youngest American man to reach the Australian Open’s third round since Pete Sampras in 1990, upsetting a racket-tossing Daniil Medvedev 6-3, 7-6 (4), 6-7 (8), 1-6, 7-6 (10-7) in a match that began Thursday night and ended in the wee hours of Friday.

The 4-hour, 49-minute contest had a bit of everything, including — hard to believe — a six-minute rain delay that interrupted play shortly before 2:30 a.m. with Tien serving at 5-all, 15-all in the fifth set. When they resumed, Medvedev broke and served for the victory at 6-5, but Tien wouldn’t cede a thing. He immediately broke back to force the concluding first-to-10 tiebreaker that he emerged to win shortly before 3 a.m., about two hours after he failed to convert his initial match point.

“Belief is a big thing in succeeding and winning, in general. I always go on the court believing that there’s a chance I can win,” said Tien, who showed up at his news conference toting a white cardboard box with a pepperoni pizza. “I wasn’t trying to think of the match as anything more important than any other match I’ve ever played. I was just going to go out there, have fun, see what I could do.”

As for the post-match snack, he said, “It was either going to be celebratory or a binge-y, like, cope. It feels better it’s more celebratory, for sure.”

The outcome was eyebrow-raising because of the wide gulf in experience and accomplishments between the two players at Margaret Court Arena. Tien is ranked 121st and owned a career Grand Slam record of 0-3 before this week; Medvedev was seeded No. 5, won the 2021 U.S. Open and was the runner-up at Melbourne Park in three of the past four years, including 12 months ago.

“It was definitely harder than maybe it could have been, but, whatever,” Tien said on court right afterward, then told the crowd: “I really appreciate all you guys staying out here. I know it’s late. I have no idea what time it is.”

Because of the time difference, the match ended at about 8 a.m. on Thursday morning back home in California, and Tien took the microphone to speak directly to his family — he said he hoped they were tuned in on TV.

“I don’t know if my parents are still watching. ... I love you guys. Thank you for always supporting me from across the world,” Tien said. “I know you guys wish you could be here. I wish you could be here, too.”

The left-handed Tien played fearlessly and almost flawlessly for stretches, surprisingly getting the better of lengthy exchanges at the baseline: Across the first two sets, he won 32 of the 51 points that lasted nine or more strokes, even coming out on top on one that went 45 shots and another that lasted 32.

This was Medvedev’s first tournament of the season — his wife recently gave birth to their second child — and the 28-year-old Russian never really displayed his best tennis.

Tien reached two junior Grand Slam singles finals in 2023, at the Australian Open and U.S. Open, and played one semester of college tennis at Southern California before turning pro that year.

He turned 19 last month, and is the youngest man from the United States to get this far at the Australian Open since an 18-year-old Sampras reached the fourth round in 1990. Sampras won the U.S. Open later that year for the first of his 14 Grand Slam titles.