Juan Martin del Potro restored parity in the Davis Cup final with a hard-fought victory over Ivo Karlovic in the second singles match in Zagreb on Friday.

Having seen Marin Cilic put Croatia 1-0 ahead in the opening match with victory over Federico Delbonis, the pressure was on Del Potro, and despite losing the second set he powered to a 6-4 6-7 (6-8) 6-3 7-5 win for Argentina.

Del Potro was quickly in the groove with a break in the opening game before a fine spell of serving that saw Karlovic struggle to make any inroads - the Croatian only winning two points on his opponents' serve in the set.

It was tight, though, as Del Potro too struggled to find a breakthrough aside from the opening game, and that continued in the second set as once again both men served with aplomb.

Karlovic - who thundered down 35 aces in the match - saved two break points in the opening game but from there on a tie-break looked inevitable. It duly arrived and it was the home favourite who - after averting two set points - levelled things up as Del Potro fired long.

Each set was proving as gripping as the last with both sides knowing a break of serve would be crucial given the ability of both men.

The eighth game of the third set was enough to give Argentina the lead once more, Del Potro converting the second break chance before serving it out.

Argentina may be the away side but the raucous fans - including Diego Maradona - who have become synonymous with their run to the final were making it feel like a home tie, no more so than in what proved to be the final set.

It followed a similar pattern to the previous three with huge cheers from both sets of supporters as each point was won, and it was the South Americans shouting the loudest as once again Del Potro found a way through.

A game later he was serving for the match and a weak backhand into the net from Karlovic secured the point, setting things up nicely for Saturday's doubles.