Novak Djokovic feels the 'pressure' after reaching Australian Open second round

Defending champion needed four sets to beat Jan-Lennard Struff, while Roger Federer and Serena Williams cruised through their openers

Novak Djokovic salutes the crowd after beating Jan-Lennard Struff in the Australian Open first round. Getty Images

Novak Djokovic admitted he was feeling the pressure after opening his Australian Open title defence with a four-set win over Jan-Lennard Struff, while Roger Federer and Serena Williams also made safe passage into the second round.

Seven-time champion Djokovic was made to sweat before beating Germany's Struff 7-6, 6-2, 2-6, 6-1 on the first day, when fears over air pollution were replaced by disruption caused by rain, which caused chaos and forced organisers to postpone a swathe of matches.

While play continued at the three stadiums courts with retractable roofs, half of the 64 scheduled matches were postponed, ensuring a big backlog for Tuesday.

Wet conditions are unusual for the Australian Open, which is more used to extreme heat and was plagued by smog from bushfires during qualifying, when players suffered coughing fits and breathing problems.

Air quality was rated 'good' as the first round started on Monday but about four hours later play was suspended on outside courts when the heavens opened in Melbourne.

World No 3 Federer was briefly hauled off court while the roof was closed on Rod Laver Arena before returning to complete a routine 6-3, 6-2, 6-2 victory over American Steve Johnson.

Williams, on the hunt for a record-equalling 24th Grand Slam title, won the first set against Anastasia Potapova in just 19 minutes as she cruised to a 6-0, 6-3 win in less than an hour.

"I feel like I can still improve and get better throughout this tournament, for sure. This is a good stepping stone for right now," Williams said.

However, Williams' elder sister Venus was ousted in stunning fashion by 15-year-old Coco Gauff, who won 7-6, 6-3 in a repeat of her first-round upset at Wimbledon last year.

Competing in the last match of the first day at Melbourne Park, Djokovic appeared to be cruising to victory before an uncharacteristic lapse saw him drop the third set. The Serbian second seed roared back in the fourth, though, to move into the second round.

"There is a lot of pressure and a lot of different emotions involved. I definitely try to remind myself to stay present and really enjoy," said the seven-time Melbourne winner, who brought up his 900th Tour-level victory.

Former US Open champion Sloane Stephens was the biggest women's casualty on day one when she crashed out in three sets to Zhang Shuai of China - her fourth first-round exit in Melbourne.

Defending champion Naomi Osaka completed her match well before the downpour as she dismissed Marie Bouzkova of the Czech Republic 6-2, 6-4 in 80 minutes, smashing one powerful serve that broke a net fastening.

"It was really tough for me trying to control my nerves," said Osaka. "It's tough to play someone you've never played before in the first round of a Grand Slam."

Later on the covered centre court, Australian world No 1 Ashleigh Barty recovered strongly from a set down to beat Lesia Tsurenko 5-7, 6-1, 6-1.

In the men's draw, Greek sixth seed Stefanos Tsitsipas, who beat Federer en route to last year's semis, crushed Salvatore Caruso 6-0, 6-2, 6-3.

Player anger over smog dominated the final days before the tournament, which is taking place after bushfires ravaged large parts of Australia, destroying thousands of homes and killing 29 people.

Tournament officials are closely monitoring pollution and will halt play and close the three main stadiums' roofs if particulate matter suspended in the air hits PM2.5 200.

In other results, Canadian 13th seed Denis Shapovalov argued furiously with the umpire over a code violation for throwing his racquet as he lost in four sets to Marton Fucsovics.

Croatian 25th seed Borna Coric was another first-round loser as he went down to experienced American Sam Querrey, while Australia's Sam Stosur bombed out against Caty McNally.

But former champion Caroline Wozniacki, playing her last tournament before retiring, safely reached the second round as she beat Kristie Ahn 6-1, 6-3.