SYDNEY - World number one Rafael Nadal believes the ATP Cup and Davis Cup should join forces.

Speaking to reporters in Sydney after Spain's 2-1 loss to Serbia in the ATP Cup Final on Sunday evening, the 19-time Grand Slam winner appeared frustrated with the scheduling of the new 24-nation round robin tournament.

"Is a long competition. Is a tough way to start the season," Nadal said.

"I think it's confusing for the spectators, and we need to be clear in our sport."

Recently sealing victory for Spain in November at the 18-nation Davis Cup competition in Madrid, the 33-year-old voiced his concerns about having two similar tournaments back-to-back.

"I think it is a great competition (the ATP Cup), 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.

"We need to find a way to fix it and we need to find a way to make a big deal with the International Tennis Federation (ITP) and Association of Tennis Professionals (ATP) to create a big World Team Cup competition, not two World Cups in one month."

"For the health of our sport and for the benefit of our sport it is, in my opinion, it's mandatory that we fix it."

Nadal will now travel to Melbourne to prepare for the opening round of the Australian Open tournament which begins on January 14.