The 17-time Grand Slam champion, who cut short his 2018 season to have surgery on a foot injury, cruised through to round two of the opening major of the year 6-4, 6-3, 7-5 against Australian wildcard James Duckworth.
Rafael Nadal and Maria Sharapova are two of the greats of the game - but both players will step onto Rod Laver Arena on Monday with an air of uncertainty surrounding them. He withdrew from a warm-up tournament in Brisbane in the first week of the season because of a muscle strain in his thigh.
Nadal, who is bidding to become the first man in the Open Era, and only the third man ever, to win each of the four Grand Slams twice, ran his opponent down with his superior court coverage and ability to hit winners under pressure.
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.
In her 15th trip to Melbourne Park and her 55th Grand Slam tournament, she's acutely aware of the toll that the sun and long early matches can have on a player's title ambitions, so she gets straight to business.
Denmark's Wozniacki broke through for her first Slam title in Melbourne a year ago, beating Simona Halep in the final, and said she was relishing defending a major for the first time.
The veteran five-time Grand Slam victor, who has struggled with injury since returning from a doping ban in 2017, showed glimpses of her old ruthless self to dispatch Dart in just 63 minutes. "I don't know my percentage, but was a lot of good positions after the first serve", he said. "I was glad that I did the right things from the beginning till the end".
"I'm just excited. I think it's a positive to be here as the defending champion".
Also advancing were 2017 U.S. Open champion Sloane Stephens, No. 11 Aryna Sabalenka, No. 19 Caroline Garcia, No. 29 Donna Vekic and No. 31 Petra Martic.
Britain's Katie Boulter, meanwhile, created a slice of history by becoming the first woman to win in a third set tiebreak -- a new rule introduced to the Open this year.
Wimbledon runner-up Kevin Anderson fired 14 aces and 47 winners to blast past Frenchman Adrian Mannarino 6-3 5-7 6-2 6-1.
Boulter beat Ekaterina Makarova 6-0, 4-6, 7-6 (6), including 10-6 in the tiebreaker. Also advancing on the men's side were No. 19 Nikoloz Basilashvili and No. 26 Fernando Verdasco.