Learner Tien, a 19-year-old American qualifier, has outlasted marathon man Daniil Medvedev in an astonishing Australian Open boilover, sending the three-time finalist spinning out of the tournament in an exhausting, early-hours epic.

With the clock ticking towards 3am on Friday, the Californian teen, who'd looked down and out after the fifth seed made one of his trademark comebacks, found remarkable mental and physical reserves to pull off a 6-3 7-6 (7-4) 6-7 (8-10) 1-6 7-6 (10-7) triumph in 4 hours 49 minutes on Margaret Court Arena.

A healthy-sized, if bleary-eyed crowd, stayed on, transfixed by the drama as world No.121 Learner - given the memorable name by his maths teacher mum - ended up the teacher, giving a lesson in indefatigability to the out-of-sorts king of the stayers, who'd earlier been in racquet-chucking mode again.

"I was definitely hoping it wasn't going to go a fifth-set breaker," smiled Tien, who'd only won his first grand slam match a couple of days earlier but is now the youngest American man in the third round in Melbourne since 18-year-old Pete Sampras in 1990. .

"Either way, just really happy to get a win. I know I made it a lot harder than maybe it could have been ... but, whatever!"

In the third-set tiebreak, the young southpaw had a match point but Medvedev snuffed it out with an ace and Tien appeared so deflated after the Russian took the set that it felt no surprise when he was outplayed comprehensively in the fourth. There looked no way back.

But Tien smiled afterwards the reason for his poor performance in that penultimate stanza was actually all down to an urgent need for a bathroom break.

"Losing the third set in a tiebreak was tough, I had match point, and it was a little bit disappointing to see a fourth set," explained Tien, the son of Vietnamese parents.

"But honestly in that fourth set, I just had to pee so bad, so I was just trying to finish it up fairly quick," he added, reducing the crowd to laughter.

"I also wanted to start the fifth serving, so I scrapped out that game and it all worked out."

It was the match of the championship, an extraordinary, fluctuating contest that, surreally, even got interrupted at 2:30am by a six-minute rain stoppage at the most critical juncture with Tien serving at five-all, 15-all in the decider.

When they returned, Medvedev broke to serve for victory at 6-5, but Tien went for broke against the overly conservative Russian, breaking back immediately and then, after trailing 6-4 in the match breaker, taking victory two hours after his first match point.

"I've no idea what time it is but I'm sure it's really late. Thanks you guys for staying out here," smiled Tien, after winning six of the last seven points and watching Medvedev float one final return over the baseline that sealed his fate.

"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," shrugged Tien later, as he chomped on a pepperoni pizza as his early-hours reward.

"It (the pizza) was either going to be celebratory or a binge-y, like, cope.

"It feels better it's more celebratory, for sure."

With AP