Milan, Metropolitan City of Milan, Italy, 11 November 2018 | AAP

Australian young gun Alex de Minaur was beaten by Stefanos Tsitsipas in the championship match of the Next Gen ATP Finals in Milan.

The second-seeded De Minaur suffered his first defeat of the tournament, succumbing 2-4 4-1 4-3(3) 4-3(3) in one hour and 40 minutes to the Greek world No.15.

Tsitsipas, 20, capped a stellar season by winning his second title of 2018, a day after he was named Most Improved Player on tour.

The pair, neither of whom qualified for the inaugural event last year, have been the most exciting breakthrough talents of 2018 so it was fitting the final pitted one against the other.

World No.31 De Minaur, the only teenager in the elite season-ending event for the best 21-and-under players, saved two match points at 3-2 down in the fourth set in the shortened format but his resistance ran out in the tiebreaker.

In the absence of Alexander Zverev, who skipped the event for the second year after qualifying for the ATP Finals, Tsitsipas was the top seed and he made no secret of how much he wanted to come out on top.

“It feels great. I’ve been playing some great tennis this week here in Milan,” Tsitsipas said.

“It was a fantastic match. I stayed calm although I had a chance twice to win the match but remained calm.

“I was aiming to win that match. I was mentally very strong and that was proven in the tie-break in the last set.”

Last year’s runner-up Andrey Rublev required five sets to defeat Spaniard Jaume Munar in the playoff for third with the Russian prevailing 1-4 4-3(4) 2-4 4-2 4-3(3).