Novak Djokovic beat Rafael Nadal before hitting the court once again within an hour to win the deciding doubles game that would secure Serbia's victory over Spain in the inaugural ATP Cup final.

Second-ranked Djokivic had a 6-2, 7-6 (4) win over top-ranked Nadal on Sunday night to level the final after Roberto Bautista Agut had given Spain a 1-0 lead by beating Dusan Lajovic 7-5, 6-1 in the first singles match.

After extending his lead to 29-26 in career head-to-heads with Nadal and extending his supremacy over the Spaniard on hardcourts, he combined with Victor Troicki in a 6-3, 6-4 win over Pablo Carreno Busta and Feliciano Lopez.

Nadal, who hasn't beaten Djokovic on a hard surface since the 2013 U.S. Open final, withdrew from the doubles match, citing fatigue. Lopez got the late call-up for the crucial doubles match.

"I have been playing a lot of tennis over the last couple of days. My level of energy is a little bit lower than usual, because I played long yesterday, very long before yesterday, very long in (Perth)," Nadal said. "So it is a team decision, and we believe in our team. That's why we had success in the past, because we were able to give confidence to the rest of the players, and we give the confidence to Feliciano and Pablo."

After playing six singles matches and two doubles matches in 10 days – on both the west and east coasts of Australia – less than two months after guiding Spain to the Davis Cup title in Madrid, Nadal urged the International Tennis Federation and the ATP to negotiate to form one world team championship.

"I think (ATP CUP) is a great competition, but at the same time – I can't change my mind that two World Cups in one month is not real. So is not possible," he said. "So we need to find a way to fix it and we need to find a way to make a big deal with ITF and ATP to create a big World Team Cup competition, not two. I think that's a confusion for the spectators, and we need to be clear in our sport."

Spain's early lead put extra pressure on Djokovic, a seven-time Australian Open champion, to keep Serbia in contention. He hadn't lost a singles match at the new 24-team tournament, and started like losing wasn't a consideration.

"We get to play a lot of exciting points. There were some incredible exchanges today," Djokovic said. "I started off the match, perfectly, really. Everything worked for me. Serve got me out of trouble in the second set."

He broke Nadal's serve in the opening game, which lasted eight minutes and held for a 2-0 lead, the game going to deuce, and then didn't concede a point on serve for his next three games, closing the set in 39 minutes with three straight aces.

Nadal, meanwhile, was struggling to hold, with Djokovic typically relentless with his returns. The Spaniard found his range on serve in the second set, though, conceding just a point in each of his first three service games.

Djokovic then had two breakpoints in the 11th game, which would have given him the chance to serve for the match, but Nadal rallied from 15-40 to hold and it was almost inevitable that the set would finish in a tiebreaker.

From 4-4, the Serbian won every point, starting the roll with a backhand winner down the line and finishing off when Nadal netted on match point.