New York, United States, 9 September 2013 | AP/

Exciting Australian tennis prospect Thanasi Kokkinakis has lost the US Open junior boys’ final in a three-setter at Flushing Meadows.

Croatian Borna Coric beat Kokkinakis 3-6 6-3 6-1 on Sunday to claim his first career Grand Slam championship.

Coric, after reaching the semifinals at the Australian and French Opens and the quarterfinals at Wimbledon, improved on those results after cementing victory in one hour and 55 minutes.

Unlike his previous two matches in which he recovered from a set down to win, Kokkinakis was the front-runner on a windy day in New York.

But as he struggled on serve and Coric came out on top of a tense 49-minute second set to level scores, the 17-year-old South Australian was unable to continue his run at Flushing Meadows.

It was his second Grand Slam final of 2013, after falling in the Australian Open final to compatriot and friend Nick Kyrgios.