Lopez is ranked 22 places below Del Potro at 31 in the world
A superb display from Feliciano Lopez saw Spain end day one of the Davis Cup final tied at 1-1 with Argentina.
David Nalbandian gave the hosts an early lead as he swept past David Ferrer 6-3 6-2 6-3 in Friday's opening singles rubber in Mar del Plata.
But Lopez came storming back to beat Juan Martin del Potro 4-6 7-6 (7-2) 7-6 (7-4) 6-3 in the second singles match.
Lopez and Fernando Verdasco will face Nalbandian and Agustin Calleri in Saturday's doubles.
Argentina are trying to win the Davis Cup for the first time, while Spain are going for a third title.
Nalbandian thrilled a raucous home crowd as he dominated the out-of-form Ferrer, breaking early in the first set and racing to victory in under two hours.
"I played well from start to finish, I was always in control of the situation and the public was incredible," said Nalbandian.
"There were some moments of tension which I managed to keep under control. It was an almost perfect game."
Ferrer admitted: "David was far superior and there's nothing more to say. The court wasn't easy but it's a tennis court like any other.
"I didn't have good sensations during the game, I wasn't up to it."
Argentina looked on course for a commanding lead when Del Potro took the opening set of the second rubber, but Lopez produced one of the performances of his career to recover.
The 27-year-old used the attacking game plan that has taken him to two Wimbledon quarter-finals, and after a fantastic season world number nine Del Potro could not cope physically.
The Argentine required treatment on his right leg in the fourth set but by then Lopez had taken the initiative, and the Spaniard closed out a fine win in three hours 19 minutes.
"I never felt this happy before in my career," said Lopez. "It's great to win a match like today, in a Davis Cup final, away from home."
Del Potro is scheduled to open the reverse singles on Sunday against Ferrer, depending on the seriousness of his thigh injury.
"The doctor could not make a diagnosis right now," said Del Potro. "I don't know what the injury is - tomorrow we'll find out what happened."
Argentina began the final as favourites in the absence of Rafael Nadal for Spain, after the world number one's season ended with a knee injury two weeks ago.
"He was calling the people on the bench in between points to find out because they cut the TV signal in the hotel he is staying at," Lopez revealed.
The tie is being played on a fast indoor court, originally chosen to try and neutralise the threat of Nadal.
Argentina are seeking their first Davis Cup title, having finished runners-up away to the United States in 1981 and Russia in 2006.
Spain won the Davis Cup in 2000 and 2004, but lost three times as visitors in 1965, 1967 and 2003 - each time against Australia.