Marin Cilic gave Croatia an early lead in the Davis Cup final with a thrilling five-set win over Argentina's Federico Delbonis in Zagreb.
Cilic, the highest-ranked player in the tie at sixth in the world, won 6-3 7-5 3-6 1-6 6-2 in the opening match of the best-of-five contest.
Juan Martin del Potro will try to draw Argentina level when he faces Ivo Karlovic in Friday's second singles.
Croatia won their first and to date only Davis Cup title in 2005.
Argentina, who beat reigning champions Great Britain in the semi-finals, hope to finally win the title after losing in four previous finals.