Spain's Rafael Nadal reacts after losing a game to Australia's Alex de Minaur during their Group D match at the United Cup tennis event in Sydney, Australia on Jan 2, 2023. (MARK BAKER / AP)

SYDNEY - Rafa Nadal likes the United Cup concept but says the mixed team tournament's format should be changed to ensure there were fewer dead rubbers going forward.

Nadal suffered his second straight loss at the $15 million event on Monday when he fell to Alex De Minaur, but both Spain and Australia were not in contention to reach the next stage after losing their opening round-robin ties against Britain.

After Britain clinched a 3-1 win over Spain in their opener, Rafa Nadal skipped the mixed doubles rubber

"The competition's great, idea is great," said Nadal, who next heads to Melbourne for his Australian Open title defence.

"It's not great that we are playing for nothing.

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"I really believe that in a group of three, the loser of the first tie needs to play with the team that hasn't played yet, because that makes the competition much more interesting," he said on Monday.

After Britain clinched a 3-1 win over Spain in their opener, Nadal skipped the mixed doubles rubber.

"With the format how it is, the mixed doubles match (against Britain) probably I would be playing, because maybe that match can make a difference even losing the tie, because we have another day," Nadal said.

Spain's Rafael Nadal plays a backhand return to Australia's Alex de Minaur during their Group D match at the United Cup tennis event in Sydney, Australia on Jan 2, 2023. (MARK BAKER / AP）

"I don't think it's good for the competition that we're playing Australia, both of us being out of the tournament. That will not happen if the country who didn't play at the beginning played with the loser of the first day.

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"All the matches are going to make sense. That's the thing I think needs improving in the competition for the future."

The inaugural edition of the United Cup is being played in Sydney, Perth and Brisbane.