Toxic air pollution from unprecedented bushfires forced a halt to practice sessions Tuesday ahead of the Australian Open in Melbourne.

Defending champion Novak Djokovic insisted there are no clear favourites for the next week's Australian Open despite his ominous start to the season.

Novak Djokovic blasted Rafael Nadal off court 6-2, 7-6 (7/4) after Roberto Bautista Agut caned Serbia's Dusan Lajovic 7-5, 6-1 in the other singles.

Rafael Nadal defeated Australian Alex de Minaur to guide Spain through their semi-final while Serbia's Novak Djokovic battled past Russia's Daniil Medvedev.

Novak Djokovic battled past fifth-ranked Daniil Medvedev 6-1, 5-7, 6-4 to send Serbia into the inaugural ATP Cup final.

No explanation was given by officials who confirmed Novak Djokovic's absence from the ATP-WTA tournament, whose women's field is headed by top-ranked Ashleigh Barty.

Novak Djokovic beat Canada's Denis Shapovalov 4-6, 6-1, 7-6 (7/4) while Dusan Lajovic recorded a 6-4, 6-2 win over Felix Auger-Aliassime.

Novak Djokovic's Serbia will take on Canada, while Spain are pitted against Belgium in the quarter-finals of ATP Cup.

On Monday, air quality in Melbourne was downgraded from 'very poor' to 'hazardous', prompting health warnings from authorities.

Rafael Nadal was in cruise control for Spain at the ATP Cup Monday but Serbia's Novak Djokovic was forced to sweat through brutal conditions for his second win in a row.

Novak Djokovic said that delaying the Australian Open should be the last option for anything, but it should be definitely considered if it affects health of players.

Rafael Nadal had little trouble with Nikoloz Basilashvili, racing to a 6-3, 7-5 victory, Novak Djokovic was given more of a test by Kevin Anderson before overcoming him 7-6 (7/5), 7-6 (8/6).

Rafael Nadal and Novak Djokovic among those looking for a winning start ahead of the first Grand Slam of the year.

Cristiano Ronaldo's towering header for Juventus against Sampdoria in a Serie A match is one of the images of sports in 2019.

While the world number two remains in a tight battle with Rafael Nadal and Roger Federer in the race for most Grand Slam titles won, Novak Djokovic believes what currently drives him in the sport is something deeper than that.

Rafael Nadal led Spain to the Davis Cup title just over three weeks ago then went on a short break before beginning his preparations for the new season.

Top-ranked Rafael Nadal and Ashleigh Barty lead the field at the 115th edition of the Australian Open from January 20-February 7.

Novak Djokovic is not one to shy away from a new challenge, though this one was a little bit of a twist.