Australian Open junior fourth seed Jakub Mensik has collapsed with full-body cramps in a dramatic finish to a brutal boys' final.

The Czech 16-year-old was serving to stay in the match at 5-6 in the deciding third set against Bruno Kuzuhara when his legs seized up, leaving him struggling to continue.

He then served successive double faults that allowed Kuzuhara to seal a 7-6 (7-4) 6-7 (6-8) 7-5 victory in an epic contest that lasted three hours and 43 minutes.

Mensik collapsed after losing championship point and received medical attention before being taken away from the court in a wheelchair.

He was unable to participate in the trophy ceremony.

"It's an unfortunate way to win it but I'm wishing Jacob a speedy recovery," top seed Kuzuhara said.

"It was a great fight. We pushed each other right until to the end.

"Every day the work we put in is paying off, as it's shown today."

Born in Brazil with Japanese heritage, American representative Kuzuhara turned the tables on Mensik at Melbourne Park after losing to the Czech in last year's US Open second round.

Kuzuhara joins the likes of countryman Andy Roddick (2000), Gael Monfils (2004), Nick Kyrgios (2013) and current world No.3 Alexander Zverev (2014) as a winner of the Australian Open boys' title.

Earlier, top seed Petra Marcinko claimed her first junior grand slam title with a 7-5 6-1 win over Sofia Costoulas in the girls' final.

In doings so, the 16-year-old Croatian extended her winning streak to 14 matches.

"The match has been really tough, playing on Rod Laver (Arena) has been a great experience," Marcinko said.

"I was kind of nervous at the beginning because it was like big stadium, first time playing a grand slam final.

"I kept calm in important moments. I kept my game through all the time. I didn't have big drops.

"Having this trophy, winning a grand slam, for me it's so big."

Marcinko follows in the footsteps of past girls' title winners Victoria Azarenka (2005), Anastasia Pavlyuchenkova (2006 and 2007) and fellow Croatian Ana Konjuh (2013).