MELBOURNE (REUTERS, AFP) - World No. 2 Rafael Nadal pulled out of Spain's opening ATP Cup tie in Melbourne on Tuesday (Feb 2) with a lower back problem, with less than a week left for the Australian Open.

The 34-year-old was due to play the second singles against Alex de Minaur in Spain's Group B tie against hosts Australia at the Rod Laver Arena but pulled out hours before his match.

"Hi all, we have decided with #TeamSpain and my team, to not play today the first match of the @ATPCup here in #Melbourne since I have a stiff low back," he said on Twitter. "Hopefully I'll be better for Thursday."

Spain, who lost to Novak Djokovic's Serbia in the final of the inaugural ATP Cup last year, will face Greece on Thursday and Nadal would hope he would be fit to play the tie and get some match practice before the Feb 8-21 Australian Open.

A spokesman for Nadal, who tied Roger Federer's 20 Grand Slam singles titles with his French Open victory last year, told Reuters that the veteran preferred "not to force too much".

Nadal was replaced by Pablo Carreno Busta for the tie but the latter began in the first singles slot, beating John Millman 6-2, 6-4 to give Spain a winning start.

The Spanish star was one of the top players who quarantined in Adelaide for 14 days after their arrival Down Under.

At the end of his isolation, he defeated Dominic Thiem 7-5 6-4 on Friday in a highly entertaining exhibition match between the men's world Nos. 2 and 3 at Adelaide's Memorial Drive Tennis Club.

Earlier, Australian Open champion Djokovic got his season off to a perfect start with a battling win over hot-shot Denis Shapovalov in the ATP Cup but world No. 3 Thiem crashed to defeat.

The Serbian world No. 1, who will be chasing a ninth Australian Open crown when the first Grand Slam of the year begins on Monday (Feb 8), was pushed hard before coming through 7-5, 7-5 against the Canadian on Rod Laver Arena.

He showed no signs of being hindered by a nasty blister on his racquet hand that troubled him at an Adelaide exhibition last week and returned 30 minutes later alongside Filip Frajinovic to play the decisive doubles rubber.

The pair beat Shapovalov and Milos Raonic 7-5, 7-6 (7-4) as the defending champions took the tie 2-1 in Group A.

Thiem's Australian Open preparations got off to a slow start as the US Open champion slumped to a 6-2 6-4 defeat by Italy's Matteo Berrettini in his opening match.

Blasting groundstrokes metres past the baseline, the Austrian was broken four times in a rusty display at Melbourne Park's John Cain Arena as world No. 10 Berrettini dominated the clash of hard-hitters.

Playing his first tour match since the ATP Finals, Thiem rallied in the second set to break Berrettini for the first time but surrendered the match the way he played it, with a big swing at a backhand that thudded into the net.

Having pushed Djokovic to five sets in the Australian Open final last year, Thiem is among the top contenders looking to dethrone the Serb at the year's opening Grand Slam.

Berrettini was thrilled to take down Thiem in the Group C tie and put Italy back on level terms with Austria at 1-1 after Dennis Novak romped past Fabio Fognini 6-3, 6-2 in the morning.

Berrettini and Fognini then combined to beat Novak and Thiem 6-1, 6-4 in the doubles for Italy's 2-1 win.

"I'm really happy for my performance," he said on court. "I'm really looking forward to playing this event and the Australian Open.

"Playing for your country is something special."

Showman Nick Kyrgios, meanwhile, pulled victory "out of a hat" in his first match for a year, rallying from a set down to get his Australian Open preparations off to a winning start.

The Australian took time to get his groove back against 209th-ranked Frenchman Alexandre Muller, losing the first set, then needing left knee treatment before knuckling down to win 3-6, 6-4, 7-6 (7-4) in the Murray River Open.

The enigmatic 25-year-old last performed in February 2020 at the Acapulco Open, before a wrist injury and then the coronavirus pandemic brought his season to a halt. He opted not to travel for the US or French Opens, but said at the weekend he was mentally refreshed and excited to be back.

"It was a tricky one, I haven't played a competitive match in about a year and I dealt with a couple of injuries when in quarantine," he said. "I was just excited to come out here, any opponent for me was going to be tricky, I just wanted to find my feet.

"I just somehow pulled it out of a hat, which seems to be the story of my career, so just happy to get through. The body feels okay."

Now ranked 47th, he will next play fellow Australian Harry Bourchier.

The Murray River Open is one of two ATP events being run concurrently at Melbourne Park ahead of next week's Australian Open.