The 38-year-old Williams, now a mother, has been stuck on 23 major triumphs since winning the Australian Open in 2017 when she beat her sister Venus.
Incredibly, Williams has lost her last four Slam finals, and each time failed to even win a set. Nevertheless, she is still a formidable presence and the woman to beat in Melbourne.
She started the season well by winning in Auckland last week - her 73rd tour victory, and her first since the 2017 Australian Open.
The 22-year-old had a turbulent 2019.
When Osaka triumphed at last year's Australian Open, she looked set to reign over the women's game.
But the Japanese then endured a slump, exiting in the first round of Wimbledon and seeing her US Open defence fall flat.
With her father standing in as her coach, Osaka roared back to the form that made her number one with back-to-back titles in Japan and China and is raring to go.
Australia's world number one will enjoy strong backing from the home crowd, but whether she thrives or shrinks in the spotlight will be key to her chances.
The 23-year-old won her maiden Grand Slam at the French Open in 2019, surging to the top of the rankings in June and staying there ever since.
However, she lost to American qualifier Jennifer Brady in Brisbane last week and the pressure to deliver for Australian fans will be immense.
28-year-old Halep will have to dispel lingering doubts about her troublesome back.
The former number one and two-time Grand Slam champion struggled towards the end of the year with the injury, which has troubled her for several years. In December was voted WTA Fan Favorite for the third time in a row.
Just 15, the American prodigy stunned five-time champion Venus Williams in the first round at Wimbledon last year, before going out in the fourth round to eventual champion Halep.
Gauff then made the US Open third round and won her maiden title in Linz in her next tournament.
At the end of 2018 she was ranked 686th in the world -12 months later she sat 68th. Expectations are high for this star in the making.
Djokovic is going for a record-extending eighth title.
He thrashed his old sparring partner Rafael Nadal in last year's final to kick off another phenomenal year where he won his fifth Wimbledon title and collected five tournament victories, taking his career tally to 77. This year could follow a similar pattern.
At 38, crowd favourite Federer still has the tools to add to his all-time record of 20 Grand Slam titles.
At this stage in his career, retirement talk is never far away but there are few signs Federer will call a halt this year as he looks to fill the only gap in his trophy cabinet - Olympic singles gold.
Top-ranked Nadal can deliver the ultimate slap to his nemesis Federer by equalling his 20 Major titles in Melbourne.
The first man to be world number one in three different decades, now 33, Nadal has been the gallant loser in two of the last three deciders but it would be just like the indomitable Spaniard to hit back with a long-awaited second Australian Open title on Feb 2.
Expectations are high for the 21-year-old Greek, who stunned Federer last year en route to the Melbourne semi-finals.
Tsitsipas had quite the season in 2019, lifting his second, third and fourth ATP trophies and beating Dominic Thiem to win the ATP Finals. He will be looking to prove he can perform and break the Big Three’s stranglehold on the Major titles.
When Kyrgios, 24, plays, controversy is never far away.
He possesses a huge forehand and serve but by general consensus, needs to curb his sparring with umpires, fans and fellow players if he is to threaten at his home Grand Slam.
He is also playing under a suspended 16-week ban, meaning the consequences will be severe if he lets his temper get the better of him.