The Croatians had won yesterday's doubles match to take a 2-1 overnight lead in the premier men's team tennis competition, and when Marin Cilic led Del Potro by two sets it seemed the hosts would go on to win only their second Davis Cup title.

However, Del Potro emphatically came from two sets down to produce a stunning comeback and take the contest to a nail-biting deciding match.

Argentina had lost in four previous finals and it looked like it would be five when Cilic raced into a 2-0 lead.

Del Potro, however, refused to be beaten and came back in stunning fashion to win 6-7, 2-6, 7-5, 6-4, 6-3 in a four hour and 53 minute epic.

The victory ended an impressive year for the 28-year-old following his return from four wrist surgeries, which also saw him win Olympic silver at Rio 2016.

"I was calm, I hoped all the time that I could win and I did it," said Del Potro.

Delbonis then followed up with a much more comfortable win over Ivo Karlovic to secure the title for the South Americans.

The 26-year-old, ranked 21 places below Karlovic, stormed to victory in just two hours and 12 minutes.

The Argentine swept past the big serving Croatian 6-3, 6-4, 6-2 in the decisive fifth contest to clinch victory.

"He was on top of every ball," said Argentina captain Daniel Orsanic.

"He played a perfect match.

"I'm surprised, in a good way. 

"I thought he couldn't win so easy."