He desperately needed a bathroom break.

So despite being up by two sets to one in the biggest outing of his young career, Tien took a calculated risk. He figured that if he tanked the rest of a set that was already running away from him—but still snatched one game—then his life would vastly improve. Not only could Tien take his nature break, he’d also serve first in the winner-takes-all fifth.

The miraculous part is that Tien, a qualifier ranked No. 121 in the world, calculated right.

Just before 3 a.m. in Melbourne, the teenage lefty from Irvine, Calif. closed out a 4-hour, 49-minute epic to take down the world’s fifth-ranked player, Daniil Medvedev, 6-3, 7-6(4), 6-7(8), 1-6, 7-6(7).

“I have no idea what time it is,” Tien said. “I’m sure it’s really late.”

Which is unusual for him, since Tien has been making a name for himself by doing things earlier than most. His victory against Medvedev on Thursday made him the youngest American man to reach the third round at a Grand Slam tournament since Donald Young at the 2007 U.S. Open. Tien is also the first American teenager to beat a top-five player at a major since Pete Sampras in 1990.

Then again, Tien has always been in a hurry. When he was first recruited to play tennis at the University of Southern California, he was on the verge of graduating high school—and just 15 years old.

“It was always just a matter of time until he took the next step,” says USC tennis coach Brett Masi.

Tien’s family had high hopes for him since birth, naming him Learner in honor of his mother, who is a teacher. (Tien’s sister is called Justice as a tribute to their father, a lawyer.) And it didn’t take long for him to prove himself to be a quick study. At 16, he won the USTA Boys’ 18s National Hard Court Championships—and then won it again the following year for good measure.

So it was no surprise that college programs were sniffing around, even if they suspected Tien might stick around for long. Masi, at USC, could tell that this wasn’t a kid who would wait very long before making the leap to the pros. In fact, once Tien joined the Trojans in early 2023, he only played 10 matches.

Still, it was enough for Masi to learn that Tien was a special type of grinder. Though he didn’t have a single standout weapon in his game, he was the kind of player who could stay in any fight.

“In every match,” Tien liked to tell him, “I’m always going to save one set point and I’m always going to save one match point.”

Except on Thursday night in Australia, Tien didn’t need to. Relying on a highly accurate first serve and his willingness to grind on every point, Tien jumped out to a 2-0 lead that knocked Medvedev onto the ropes. In the third, he even had a match point to end his remarkable evening much earlier. Only then did Medvedev claw his way back into the match.

“Losing the third was tough,” Tien said. “I know I made it a lot harder than it could have been. But you know, whatever.”

Tien isn’t the only teenager lighting up this tournament. As the generation that was supposed to supplant the Big Three of Novak Djokovic, Rafael Nadal, and Roger Federer inches toward 30, a fresh crop of youngsters is ready to crash the party. Before Tien even took the court against Medvedev, there had already been upset victories by Brazil’s Joao Fonseca over No. 9-seed Andrey Rublev and the Czech Republic’s Jakub Mensik over No. 6-seed Casper Ruud. That makes this year’s Australian Open just the third Grand Slam event to see three teenagers knock off Top-10 players since 1973, according to Opta Sports.

Neither Fonseca nor Mensik, however, left things quite as late as Tien.

By the time he eventually finished the job, in the middle of the Australian night, it was the sixth-latest finish of a major match in men’s tennis history. Tien knew it was unlikely he’d get to bed before 5 a.m. But back home on the West Coast, where his most loyal fans had stayed up all night glued to the television, it was time for breakfast.

“I’m so tired,” Tien’s father Khuong texted Masi. “I’m going to sleep until his next match.”

Write to Joshua Robinson at Joshua.Robinson@wsj.com