Lleyton Hewitt (Aus), 34, ranked 306th
Best Open result: Finalist (2005)
This Open will bring down the curtain for the former world No. 1 and two-time Grand Slam winner, who will retire after his 20th appearance at Melbourne Park. The home favourite will be hoping for a deep run.
The warhorse, renowned for his all-or-nothing fighting spirit, begins against compatriot and fellow wild card James Duckworth, 23, in a likely night match on Rod Laver Arena, with the threat that every match could be his last.
"To tell you the truth, I don't know how it feels," he told reporters when asked what it was like to be one defeat from the end of his career.
"A tad strange feeling, but I'm trying to soak it up as much as possible."
Milos Raonic (Can), 25, ranked 14th
Best Open result: Quarter-finals (2015)
The 13th seed has been knocking on the door to win a Major and will head into Melbourne Park high on confidence.
Raonic - the youngest player inside the top 15 - defeated 17-time Grand Slam champion Roger Federer in the Brisbane final last week, underlining his ability to beat the world's best.
More importantly, the big server finally looks healthy again after undergoing foot surgery in May last year, when he reached a career high No. 4 ranking.
Jack Sock (USA), 23, ranked 26th
Best Open result: Second round (2014)
Austria's Dominic Thiem, 22, and Australian Bernard Tomic, 23, are the only players above Sock in the world rankings who are younger than the American.
With the Big Five of Novak Djokovic, Andy Murray, Roger Federer, Stan Wawrinka and Rafael Nadal dominating in recent years, the young guns have struggled to impose themselves.
But Sock notched an impressive win over world No. 8 David Ferrer to reach the ASB Classic final in Auckland last week.
That was his 11th win over a top 20 player since the start of the 2015 season - the most of any player not ranked in the top 20.