Two-time former champion Naomi Osaka is also in action on day three of the first Grand Slam of the year, as is highly promising Brazilian teenager Joao Fonseca.
Sinner was a comfortable winner over Alexander Zverev in last year’s final and if he wins the tournament again, he would join Novak Djokovic as the only men in the Open era to win three successive Melbourne crowns.
The Italian second seed faces French world number 93 Hugo Gaston at Rod Laver Arena in the first match of the night session.
The chief threat to his Australian Open dominance is top-ranked Carlos Alcaraz, who is bidding to win the tournament for the first time and seal a career Grand Slam of all four majors at just 22.
The Spaniard eased through his opening match on Sunday in straight sets.
Following defeat to great rival Alcaraz in the US Open final, the 24-year-old Sinner vowed to add more variation to his game.
“We worked a lot on trying to make the transition to the net. The serve we changed a couple of things,” he said.
“When you are at the top level, these are the small details that make the difference.”
Also playing Tuesday in the first round are Italian fifth seed Lorenzo Musetti, all-action eighth seed Ben Shelton, and his fellow American and ninth seed Taylor Fritz.
There will be interest in the 19-year-old Fonseca, who was tipped on the eve of the tournament by Roger Federer to one day challenge the duopoly of Alcaraz and Sinner.
The 28th seed plays another American in Eliot Spizzirri.
Keys stunned world number one Aryna Sabalenka in the women’s final 12 months ago but the American failed to push on and did not win another event all year.
The Melbourne ninth seed was bundled out in the quarter-finals of both her warm-up tournaments, first to Sabalenka and then to Canadian 19-year-old Victoria Mboko.
The 30-year-old Keys is trying to enjoy the added pressure that comes with being the holder.
“I’m really trying to just kind of embrace that and take it in and soak it in,” said Keys, who opens the day’s play on Rod Laver Arena against Ukraine’s Oleksandra Oliynykova.
Former world number one Osaka, the 16th seed, features in the final encounter of the day on centre court, against Croatia’s Antonia Ruzic.
The 28-year-old Japanese has struggled to get back to the form and fitness that took her to the Australian Open title in 2019 and 2021.
Top-10 seeds Elena Rybakina and Belinda Bencic also begin their campaigns.