It was a meek exit for Russia’s former world no 1 Medvedev, who has lost three of the last four finals at Melbourne Park, including to Jannik Sinner in 2024.
The 28-year-old Medvedev needed five sets to beat the 418th-ranked Kasidit Samrej earlier in the week, smashing his racquet and a net camera in a rage at one point.
He started more serenely against Tien but errors crept into his play and Tien shook off any early nerves by clinching the first set in 47 minutes.
Tien showed his mettle and his pedigree in the eighth game by prevailing in one gruelling 32-shot rally.
The 2021 US Open champion Medvedev and Tien went to a second-set tie-break, where the Russian was left exasperated by a double foot fault on his way to losing the set.
The third set again went to a tie-break, where Medvedev pulled out an ace under excruciating pressure to save match point and then clinched the set at the third time of asking to come back from the brink.
Suddenly it was all Medvedev and he raced through the fourth set to tee up a decider.
Tien appeared to have run out of steam.
But from somewhere he located some extra reserves of energy and the duo exchanged breaks early in the fifth set.
There was still time for another twist at 5-5 and 15-all on Tien’s serve when a shower came, briefly holding up proceedings.
They exchanged breaks of serve to go into a 10-point tie break, where there was again nothing between them until Tien finally sealed the deal.
Tien is a known talent in tennis circles.
He was defeated by fellow teenage prospect Joao Fonseca – who stunned ninth seed Andrey Rublev in round one in Melbourne – in last month’s NextGen final for up-and-coming players.