World No 1 Rafael Nadal has called on tennis administrators to create one "big world team cup competition" instead of confusing fans by staging the ATP Cup and Davis Cup within two months of each other.
Nadal won the International Tennis Federation's revamped Davis Cup competition with Spain in November and lost Sunday's final of the inaugural ATP Cup to Novak Djokovic's Serbia.
"I think it's a great competition but ... I can't change my mind that two world cups (within two months) is not real," Nadal told reporters.
"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 world cups.
"I think that's a confusion for the spectators, and we need to be clear in our sport."
Other top players, including Djokovic, have previously backed the merger of the two team events.
Djokovic, De Minaur withdraw late from inaugural Adelaide tournament
Australia's top-ranked men's player Alex de Minaur has withdrawn from the inaugural Adelaide International due to a rib injury, organisers said on Monday, joining world number two Novak Djokovic as late withdrawals.
Women's world No. 10 Kiki Bertens of the Netherlands also withdrew on Monday with an Achilles' injury.
Both de Minaur and Djokovic had heavy workloads during the ATP Cup tournament, a new teams event introduced this year that was played in Perth, Brisbane and Sydney last week.
Adelaide organisers said on Twitter 20-year-old de Minaur, who was the top seed in Adelaide after Djokovic's withdrawal, had re-aggravated a rib injury during the ATP Cup.
De Minaur led Australia into the semi-finals of the tournament while Djokovic helped Serbia win the final on Sunday after he beat Spain's Rafa Nadal in the singles then partnered Victor Troicki in the deciding doubles tie.
Organisers did not give a reason why Djokovic withdrew at the weekend.
"We know how disappointed Novak is about not being able to play in Adelaide this year and we hope to welcome him to the tournament next year," Adelaide International Tournament Director Alistair MacDonald said in a statement.
"We understand his decision and wish him the very best of luck for the ... upcoming Australian Open."
The Adelaide tournament, which kicked off on Sunday, is a combined ATP and WTA event that replaces the Sydney International, which was dropped from the calendar to accommodate the ATP Cup.
World number one Ash Barty and Wimbledon champion Simona Halep headline the women's draw.