Maria Sharapova has the second-best record among active players in first-round matches at the majors, and she showed why with a ruthless 6-0, 6-0 win over Harriet Dart in Melbourne.
It was the South African's first win in Melbourne since 2015.
There were also questions about the 2017 US Open winner's form coming into the tournament after poor performances this month at warm-ups in Brisbane and Sydney.
17-time Major champion Rafael Nadal beat 238th-ranked James Duckworth 6-4, 6-3, 7-5 to advance to the second round of the Australian Open.
Sharapova, a victor at Melbourne Park in 2008 who has failed to impress at Grand Slam level since returning from a doping ban in 2017, signalled her intent with a 6-0, 6-0 thrashing of Britain's Harriet Dart, who ended the match in tears.
Ninth seed John Isner was stunned by his American compatriot Reilly Opelka, who secured a 7-6 (7-4) 7-6 (8-6) 6-7 (4-7) 7-6 (7-5) victory in a tight encounter between the towering pair that looked set to go the distance.
Duckworth was aggressive early, including serving and volleying, but a double fault handed Nadal a break in the opening game.
Nadal has only lost twice in the first round at Grand Slams - to Steve Darcis at 2013 Wimbledon, and to Fernando Verdasco here in 2016 - and is aiming to be the first man in the Open era two win all four majors at least twice.
"I mean, there is no time for that, I'm sorry to say", the 2008 victor said.
"But, you know, I felt like I did all the right things today in order to get through that match". Since the beginning of my career I always had the same team, and with Kia it's the same.
Wozniacki sealed the match with a forehand victor on her third matchpoint and will next meet Sweden's Johanna Larsson, who progressed after Vera Lapko of Belarus retired while trailing 7-6 (5) 3-0.
It's been a poor start to the season for Ostapenko.
Although she had two break points in the third game of the second set, Watson failed to take advantage and ended up losing in just over an hour.
Boulter beat Ekaterina Makarova 6-0, 4-6, 7-6 (6), including 10-6 in the tiebreaker.
The 22nd-seeded Ostapenko, who also advanced to the semifinals at Wimbledon past year, had beaten Sakkari in three sets in their only previous meeting at Charleston in 2017.
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