Spanish captain Jordi Arrese sprang a major surprise by dropping Juan Carlos Ferrero from the side to face the United States in the Davis Cup final.
DAVIS CUP FINAL
Spain v USA
Estadio Olimpico, Seville
Carlos Moya will open play against Mardy Fish at 1100 GMT on Friday, followed by Andy Roddick against Ferrero's replacement Rafael Nadal.
Ferrero has struggled with injuries all year but insists he is "100% fit".
Nadal and Tommy Robredo meet Mike and Bob Bryan in Saturday's doubles, before the reverse singles on Sunday.
"Juan Carlos Ferrero isn't in perfect form and Rafa has been great, and he's left-handed, which gives the Americans something different to deal with," said Arrese.
Ferrero admitted: "It was certainly a surprise not to be picked," said "Physically I am at 100% and I have had three days of good training.
"Obviously I am not jumping up and down with happiness, but you have to take things as they come."
Ferrero's replacement was equally taken aback at his selection.
"I'm a bit surprised, but I feel I am ready," said Nadal.
"It will be complicated against the world number two and I will have to be at my best."
The team captains can change the line-up an hour before the match and Ferrero has not ruled out returning for Sunday's singles.
"Of course I want to see myself on the court on Sunday," he said.
Roddick said: "Personally, I was expecting to play Ferrero.
"But at the same time I think our team knows that they have four very capable singles players and that's one of their strengths."
And US captain Patrick McEnroe warned against any complacency.
"It wasn't a surprise," he said. "Nadal is going to be a great player, but we have a great opportunity here to show what we can do."
Ranked 51st in the world, the 18-year-old Nadal has far less experience than former French Open champion Ferrero, who was part of Spain's last Davis Cup winning side in 2000.
However, the fiesty Nadal has impressed in his first full year on the tour and is one of only four players to have beaten Roger Federer in 2004.
Nadal, who is Spain's youngest ever Davis Cup player, also won the deciding match against the Czech Republic in February, and clinched victory in the semi-finals against France.
Spain remain favourites to take the title thanks in large part to the slow clay court that has been laid in Seville's revamped Olympic Stadium.
The atmosphere should also prove daunting for the visiting Americans, with organisers expecting the largest crowd ever to attend a "sanctioned" tennis event at 26,000.