Argentina kept alive their Davis Cup final hopes with a crushing doubles victory over Spain in Seville.
Trailing 2-0 after the first day's singles, Argentina's David Nalbandian and Eduardo Schwank saw off Spain's Feliciano Lopez and Fernando Verdasco 6-4 6-2 6-3 in Saturday's only rubber.
But the visitors will need to win both of Sunday's singles matches to stop the hosts winning a fifth Davis Cup.
Juan Martin Del Potro will face Rafael Nadal in the first match of the day.
Juan Monaco is then scheduled to meet Spanish world number five David Ferrer in the fifth rubber.
However, Nalbandian played so well he could be drafted in by Argentina skipper Tito Vazquez for the final match.
Despite being faced by a noisy Spanish crowd, Nalbandian and Schwank set about their task with confidence, claiming the only break of a closely-fought opening set to establish an early advantage.
The second set was one-sided and, while four-time champions Spain showed some spirit in the third, Argentina wrapped up an impressive victory in one hour and 59 minutes.
"We played a great match from the beginning and it was unbelievable for us," Nalbandian told Davis Cup Radio.
"We didn't lose concentration at any moment and this was very important for us. We needed to keep alive in the tie and we still have a chance."
Vazquez said: "They won the match from beginning to end so I am very happy.
"We were hoping that Del Porto would win on Friday and put us in a good position, but if anyone is capable of beating Nadal, that is Juan Martin."
Lopez, the first Spanish player to compete in four Davis Cup finals, said it had been his worst performance in the competition.
"I didn't feel comfortable with the serve and with the return," the 30-year-old said. "But I can't torture myself. Things didn't work out. We have to trust David and Rafa to give us good news."
World number two Nadal thrashed Monaco on Friday to secure his 15th straight Davis Cup singles victory on clay, while Del Potro lost a five-hour epic against Ferrer.
"We're optimistic," Spain captain Albert Costa. "Tomorrow [Saturday] I have Rafa and David and we're going to win one of those two points."
The final is a repeat of 2008, when Spain won on a hard indoor surface in Argentina despite missing Nadal because of injury.