Zagreb - Croatia won the opening singles of the Davis Cup final after Marin Cilic struggled past Argentina's Frederico Delbonis 6-3, 7-5, 3-6, 1-6, 6-2 on Friday.
Cilic, the 2014 US Open champion, looked like cruising to the expected easy victory until Delbonis broke the Croat twice in the third set, which ended on a Cilic double fault.
"I struggled on my return," said Cilic, the No 6 ranked player. "I never said the match would be easy. After all, this is the final."
Delbonis started the fourth set 3-0 with a double break, and Cilic conceded the set on serve by netting an easy smash.
But after a bathroom break, Cilic came back renewed and broke Delbonis to start the fifth set on the indoor hard court at Arena Zagreb.
Cilic suddenly rediscovered his serving groove, and didn't give Delbonis a chance.
Cilic broke again in the seventh game, and served out after three and a half hours - two hours longer than most expected.
Watched by 12 000 fans, including thousands of Argentinians who outshouted the home supporters, Cilic served 17 aces in the match. Among the support was former Argentine soccer star Diego Maradona.
"His (Cilic) serve was the key," Delbonis said. "In the first two sets, he was serving unbelievably. When I started making some returns, things changed."
Ivo Karlovic, at 37 the oldest ever singles player to take part in the final in almost a century, was playing Argentina's Juan Martin del Potro, the 2009 US Open champion, in the second singles.
Argentina is the only nation in the Davis Cup's 116-year history to have reached four finals without winning the title.
Croatia is targeting its second title after beating Slovakia in the final in 2005.