Rafael Nadal didn't make it for Spain's ATP Cup opener because of a sore back, deciding it wasn't worth the risk a week ahead of the Australian Open.

Novak Djokovic did double-time for defending champion Serbia, meanwhile, winning singles and doubles matches Tuesday to ensure a 2-1 comeback win over Canada.

The top two players in men's tennis took different approaches to their fine-tuning at the start of a hectic week of preparation for the season's first major.

Both spent 14 days in hotel quarantine — as did all incoming players — because of Australia's strict border regulations amid the COVID-19 pandemic, and both played in an exhibition event last Friday. Nadal played more of a hand in that, with Djokovic limiting his time on court because of a blister.

Djokovic was in his element on Rod Laver Arena while Serbia fans chanted and waved their national flag as he beat Denis Shapovalov 7-5, 7-5 to force a deciding doubles match, then combined with Filip Krajinovic for a 7-5, 7-6 (4) win over the Canadian pair.

Last week in Adelaide, Djokovic spoke about feeding off the energy of the crowds.

Back at Melbourne Park, where he has won a record eight Australian Open titles, he had plenty of energy to consume.

The large and loud band of Serbia supporters was a feature of Djokovic and his team’s win in the inaugural ATP Cup last year, which culminated with a victory over Nadal and Spain.

“It’s a pleasure to see them in such big numbers. Whenever they can, they show up,” Djokovic said. “Last year it was amazing. That was probably the best support outside of Belgrade that we ever had.

“We need that. In this kind of competition, it’s necessary.”

The second-ranked Nadal posted a statement on Twitter before his scheduled night match against Australia’s Alex de Minaur saying he had a stiff lower back and “hopefully I’ll be better for Thursday.”

Spain didn’t miss a beat, with Pablo Carreno Busta and Roberto Bautista Agut winning their singles matches to secure a win over Australia in Group B.

The ATP Cup, a 12-team competition in 2021, is one of the six tournaments crammed into the schedule this week to give players as much chance as possible to prepare for the Australian Open, which has been delayed three weeks from its usual January time slot.

With coronavirus cases under control, the government is allowing crowds of up to 25% capacity for the tune-up events for the season's first tennis major, rising to 50% from the start of the Australian Open on Monday.

That's something the players haven't experienced since the pandemic started to change the international sports landscape.

Matteo Berrettini said having the crowd back helped during his 6-2, 6-4 upset win over U.S. Open champion Dominic Thiem in Italy's 2-1 victory over Austria in Group C. Russia opened Group D with a win over Argentina.