After losing the opening game, Williams won 10 games in a row and beat Laura Siegemund 6-1, 6-1 on the first day of the tournament in Melbourne. Monday's victory marked the start of Williams' latest bid for a record-tying 24th major title.
Williams, the No. 10 seed, took the court in a colorful, one-legged catsuit, and her game looked flashy too. She lost only nine points on her serve and hit 16 winners. She saved one of her best shots for the final game, racing forward to flip an off-balance backhand off her shoestrings for a crosscourt winner.
"This was a good start -- vintage Rena," said Williams, who became the only woman since the Open era began in 1968 to play 100 Australian Open matches (88-12 record). "It's definitely good. I'm pretty good at pacing myself at a Grand Slam."
For the past four years, Williams has been trying to equal Australian Margaret Court's record for major titles. Williams' most recent Grand Slam championship came at Melbourne in 2017.
"I've had a ton of pressure, and now I don't feel it anymore,'' she said. "It's like a huge relief. I think I was just looking at it all the wrong way in the past, and I feel totally different about it now."
Williams wore a colorful one-legged catsuit that she said was inspired by former Olympic sprinter Florence Griffith Joyner, and her game looked flashy, too. She lost only nine points on her serve and hit 16 winners, and showed no signs of the right shoulder issue that led her to pull out of a tuneup tournament.
No. 3 Osaka, who won the title in Melbourne two years ago, played the first match of the tournament in Rod Laver Arena and beat Anastasia Pavlyuchenkova 6-1, 6-2.
The first tourney of this year's Grand Slam season began after a three-week delay because of the coronavirus pandemic.
"Physically, I feel like everyone -- their bodies are shocked, coming in and playing so suddenly after such a long break," Osaka said. "I think we're all getting used to it, and everyone is just happy to be here."
Osaka drew a potentially tough opening opponent in Pavlyuchenkova, a Russian player ranked 39th who reached the quarterfinals in Melbourne in three of the past four years. But Osaka breezed through the first set in 21 minutes and barely slowed after that; she finished with more winners than errors and lost only five points on her first serve.
Osaka has won 15 consecutive matches, including the US Open final in September, since her most recent loss in a Fed Cup match a year ago.
The win ended Venus Williams' four-match losing streak at majors, the longest of her career. The victory came in Williams' 88th Grand Slam tournament, a women's record.
At 40, Williams is the oldest woman in this year's draw and just the sixth player in her 40s to compete at the Australian Open.
"I like my job,'' she said. "No matter what happens to you in life, you always hold your head up high and give 100 million percent. And that's what I do every single day, and that's something I can be proud of.''
The second-seeded Romanian looked in fine form as she moved her opponent around the Rod Laver Arena court apparently at will and seized her break-point opportunities with alacrity.
Canadian eighth seed Andreescu was made to work hard on her return to competition after 15 months, the 2019 U.S. Open winner digging deep to see off Mihaela Buzarnescu of Romania 6-2, 4-6, 6-3.
The 20-year-old, playing her first match since retiring from the 2019 WTA Finals in Shenzhen with a knee injury, moved assuredly on court and showed no signs of discomfort against the world No. 138.
Canadian Rebecca Marino, a former top-40 player competing in a Grand Slam event for the first time in eight years, beat wild-card entry Kimberly Birrell 6-0, 7-6 (9). Marino had been sidelined by depression and a serious foot injury.
"I still have my big serve and big forehead. That hasn't changed that much,'' Marino said. "And I have a confidence that I know I belong again.''
Up to 30,000 fans -- about 50% of capacity -- will be allowed on the tournament site, but seats were mostly empty for the start of play on a cool morning.
"Listen, this is amazing," Serena Williams said. "Last year was very scary for the world. To be able to do what I love and be able to come out and compete ... it makes me appreciate the moment even more."
The Associated Press and Reuters contributed to this report.