De Minaur was scheduled to take on world No.2 Nadal in Melbourne on Tuesday night, but the 20-time grand-slam winner withdrew just before the tie started with a stiff lower back.

It meant Spain's No.3 ranked player - world No.16 Pablo Carreno Busta - was called up to play John Millman in the first singles match.

Millman showed plenty of fighting spirit but he was no match for Carreno Busta in a match that lasted just 77 minutes, with the Spaniard winning 6-2 6-4.

World No.23 de Minaur needed to beat world No.13 Roberto Bautista Agut to keep the tie alive, and he made a scintillating start to race out to a 5-1 lead.

The 21-year-old was broken twice in the next three games but was still able to seal the first set.

But from there Agut took control, snaring early breaks in each of the next two sets to secure the 4-6 6-4 6-4 win in two hours and 20 minutes.

"Going in you're expecting to play Rafa, but we got told at the warm-up, and you just go reset and focus on your new opponent," de Minaur said of the late opponent switch.

"It didn't affect me too much. I mean, I had a game plan, was playing some good tennis and it kind of slipped away today."

Millman said Carreno Busta was too good for him on the night.

"You've got to give credit where credit is due," Millman said.

"I didn't play that great but Pablo was very good. I probably needed slightly better court position, needed to execute a couple of the bigger points a little better.

"And you know, that's maybe not abnormal due to the fact that it's your first match back after a bit of a break. Sometimes those little one per centers just aren't there. That was the case for me."

Spain wrapped up the tie 3-0 after Carreno Busta and Marcel Granollers beat John Peers and Luke Saville 6-4 7-5 in the doubles.

In Tuesday night's other tie, Russia beat Argentina 2-1 after Andrey Rublev beat Guido Pella 6-1 6-2, and Daniil Medvedev defeated Diego Schwartzman 7-5 6-3.

Australian Associated Press