Melbourne - Serena Williams and Roger Federer made winning starts before heavy rain caused chaos on day one of the Australian Open on Monday, further disrupting the Grand Slam after a build-up hit by haze from raging bushfires.
Hazardous smog left several players with coughs and breathing difficulties during qualifying last week, prompting speculation about whether the year's first tennis Major would be delayed.
Air quality was rated as 'good' as the tournament started on schedule but play was suspended on outside courts about four hours later as a downpour deluged Melbourne Park at about 15:00 (06:00 SA time).
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.
Play also continued under the retractable roofs of Margaret Court Arena and Melbourne Arena, but was impossible elsewhere as the heavy rain persisted.
More rain is forecast for Tuesday, further threatening a tournament more often associated with extreme heat at the height of the Australian summer.
Officials said play would resume on the outside courts once the rain stops, but at least 18 of the 64 scheduled matches were called off for the day.
Defending champion 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."
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 evening session on the covered centre court, Australian world number one Ashleigh Barty plays Ukraine's Lesia Tsurenko and men's defending champion Novak Djokovic takes on Jan-Lennard Struff.
Player anger over smog dominated the final days before the tournament, which is taking place after deadly bushfires ravaged large parts of Australia.
Slovenia's Dalila Jakupovic was forced to retire from her qualifier after a coughing fit, while Britain's Liam Broady claimed "multiple" players needed asthma medication.
Federer was one of the players who criticised a lack of communication from tournament officials, who were forced to suspend practice and delay some qualifying matches last week.
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 with the umpire in a tempestuous defeat as he lost in four sets to Marton Fucsovics after earning a code violation for throwing his racquet.
Croatia's 25th seed Borna Coric was also an early casualty as he went down in three sets to experienced American Sam Querrey.
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.
Collated results on day one of the Australian Open Grand Slam at Melbourne Park on Monday:
Matteo Berrettini (ITA x8) bt Andrew Harris (AUS) 6-3, 6-1, 6-3
Ricardas Berankis (LIT) bt Roberto Carballes (ESP) 6-4, 6-2, 6-2
Sam Querrey (USA) bt Borna Coric (CRO x25) 6-3, 6-4, 6-4
Guido Pella (ARG x22) bt John-Patrick Smith (AUS) 6-3, 7-5, 6-4
Gregoire Barrere (FRA) bt Mohamed Safwat (EGY) 6-7 (8/10), 7-6 (7/1), 6-4, 7-6 (7/5)
Marton Fucsovics (HUN) bt Denis Shapovalov (CAN x13) 6-3, 6-7 (7/9), 6-1, 7-6 (7/3)
Grigor Dimitrov (BUL x18) bt Juan Ignacio Londero (ARG) 4-6, 6-2, 6-0, 6-4
Daniel Evans (GBR x30) bt Mackenzie McDonald (USA) 3-6, 4-6, 6-1, 6-2, 6-3
Yoshihito Nishioka (JPN) bt Laslo Djere (SRB) 6-4, 3-6, 6-2, 7-6 (7/1)
Roger Federer (SUI x3) bt Steve Johnson (USA) 6-3, 6-2, 6-2
Stefanos Tsitsipas (GRE x6) bt Salvatore Caruso (ITA) 6-0, 6-2, 6-3
Philipp Kohlschreiber (GER) bt Marcos Giron (USA) 7-5, 6-1, 6-2
Novak Djokovic (SRB x2) bt Jan-Lennard Struff (GER) 7-6 (7/5), 6-2, 2-6, 6-1
Zhu Lin (CHN) bt Viktorija Golubic (SUI) 4-6, 6-1, 7-6 (10/5)
Julia Georges (GER) bt Viktoria Kuzmova (SVK) 6-1, 6-2
Petra Martic (CRO x13) bt Christina McHale (USA) 6-3, 6-0
Ekaterina Alexandrova (RUS x25) bt Jil Teichmann (SUI) 6-4, 4-6, 6-2
Barbora Krejcikova (CZE) bt Kaia Kanepi (EST) 7-6 (7/3), 2-6, 6-3
Paula Badosa (ESP) bt Johanna Larsson (SWE) 6-1, 6-0
Petra Kvitova (CZE x7) bt Katerina Siniakova (CZE) 6-1, 6-0
Naomi Osaka (JPN x3) bt Marie Bouzkova (CZE) 6-2, 6-4
Zheng Saisai (CHN) bt Anna Kalinskaya (RUS) 6-3, 6-2
Sorana Cirstea (ROU) bt Barbora Strycova (CZE x32) 6-2, 7-6 (7/5)
Ann Li (USA) bt Lizette Cabrera (AUS) 7-6 (7/4), 7-6 (12/10)
Sofia Kenin (USA x14) bt Martina Trevisan (ITA) 6-2, 6-4
Caroline Wozniacki (DEN) bt Kristie Ahn (USA) 6-1, 6-3
Tamara Zidansek (SLO) bt Han Na-lae (KOR) 6-3, 6-3
Serena Williams (USA x8) bt Anastasia Potapova (RUS) 6-0, 6-3
Ashleigh Barty (AUS x1) bt Lesya Tsurenko (UKR) 5-7, 6-1, 6-1
Coco Gauff (USA) bt Venus Williams (USA) 7-6 (7/5), 6-3
Caty McNally (USA) bt Samantha Stosur (AUS) 6-1, 6-4
Zhang Shuai (CHN) bt Sloane Stephens (USA x24) 2-6, 7-5, 6-2