Federer, Williams duck downpours at wet, wet, wet Australian Open

Third seed Roger Federer made short work of Steve Johnson in the Australian Open first round. (Photo: AFP/William West)

Defending champion Djokovic was made to sweat before beating Germany's Jan-Lennard Struff in four sets on the first day, when fears over air pollution were replaced by disruption caused by rain.

While play continued at the three stadiums 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 number three 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 (7/5), 6-3 in a repeat of her first-round upset at Wimbledon last year.

In the late match, defending champion Djokovic dropped his first set since 2006 in the opening round before recovering to beat Jan-Lennard Struff 7-6 (7/5), 6-2, 2-6, 6-1.

"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.

SHAPOVALOV RACQUET ROW

Defending champion Naomi Osaka was done 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 number one 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.