Croatia's Marin Cilic outlasted David Nalbandian for a 5-7, 6-4, 4-6, 7-6 (7-2), 6-3 victory in the day's first match. Del Potro evened the best-of-five series with a 6-2, 7-5 (9-7), 6-1 decision over Ivo Karlovic.
The countries will be looking to gain the series lead Saturday when Cilic and Lovro Zovko are scheduled to take on Nalbandian and Eduardo Schwank in a doubles match.
There are two singles matches set for Sunday, with del Potro and Cilic playing before Nalbandian goes against Karlovic.
It will be interesting to see if Cilic and Nalbandian play their three scheduled matches this weekend because their first lasted 5 hours, 9 minutes. It wasn't pretty, either, with 14 double faults (11 by Nalbandian) and a combined 141 unforced errors over 379 points.
Both had five breaks but Cilic was able to get the only one of the fifth set, saving three break chances for Nalbandian.
Del Potro only saw one break point all day in his win. He had more winners than unforced errors (31-30) and won three-quarters of the points on serve.
The survivor of the Croatia-Argentina series moves to the Sept. 14-16 Davis Cup semifinals against the winner of the series between the Czech Republic and Serbia. That best-of-five was also 1-1 after Friday's play.