Nadal routed Richard Gasquet 6-3, 6-0, 6-1 on his favorite clay surface, and David Ferrer cruised past Gilles Simon 6-1, 6-4, 6-1 to put Spain ahead 2-0 at Cordoba's bullring in the best-of-five series.
Defending champion Serbia, playing without Djokovic, trailed Argentina 2-0 in Belgrade in the other semifinal. Djokovic has an ailing back and was a late withdrawal from opening singles. His replacement, Viktor Troicki, lost to David Nalbandian 6-4, 4-6, 6-2, 6-3. Janko Tipsarevic was beaten by Juan Martin del Potro 7-5, 6-3, 6-4 in the other match.
Argentina needs one more point to reach its first final since 2008. Serbia has never successfully come back after dropping the opening singles.
After beating Gasquet in a little more than two hours, Nadal complained again about the tennis schedule and didn't rule out a strike if there wasn't a resolution.
"We don't want to get there. We want to play," Nadal said. "But if it's a fight about something that we think is fair, something would have to happen."
Nadal broke seven times, improving his Davis Cup singles record on clay to 13-0.
"I'm close to the end of my energy," Nadal said. "I'm not in perfect condition physically, I know that. If the match stays longer it (would) be very difficult for me."
If Spain advances, Nadal said he would prefer to face Argentina in the final. The match would be in Spain, and he'd avoid meeting Djokovic, who is 6-0 against him in finals this year.
Serbia managed to get through its semifinal against Czech Republic last year when Djokovic withdrew from the opening singles in similar circumstances. At that time, he had just lost the U.S. Open final to Nadal.
Djokovic, who was treated for a back injury during the U.S. Open final, arrived in Belgrade on Thursday. Troicki made 58 unforced errors against Nalbandian, who took his Davis Cup singles record to 22-5.
"Djokovic's absence helped us for sure," Nalbandian said. "I found out some 25-30 minutes before the match that he will not play. But I was prepared to play either him or Troicki. It did not matter."
Djokovic posted on Twitter: "Congratulation to them for today. They played great matches! We don't give up,hoping we can win doubles!"
Serbia captain Bogdan Obradovic said Djokovic could play in doubles Saturday.
In Sydney, Roger Federer made a sluggish start before defeating Lleyton Hewitt 5-7, 7-6 (5), 6-2, 6-3 to put Switzerland in a 1-1 tie with Australia in a World Group playoff. Earlier, Bernard Tomic beat Stanislas Wawrinka 4-6, 6-4, 6-3, 6-3.
Federer said he was feeling the effects of his five-set loss to Djokovic in the U.S. Open semifinals and felt uncomfortable on an "old-school" grass court.
"I all of a sudden realized this was much more difficult than I expected," Federer said.
The winners of the eight playoffs will play in the 16-team World Group in 2012.