First up, the 36-year-old Swiss will play Ajaz Bedene of Slovenia.
"There was a doubt, especially after I pulled out of the first week of the year", he admitted.
Last year's quarterfinalist Johanna Konta is in the same quarter as Halep and faces American Madison Brengle first up.
Novak Djokovic returned to competitive tennis with a bang on Wednesday, bringing a six-month spell on the sidelines to an end with a thumping 6-1 6-4 win over world number five Dominic Thiem at the Kooyong Classic exhibition tournament.
Players could forfeit their prize money if they retire from first-round singles matches at the Australian Open, under new rules aimed at limiting the early injury withdrawals which have long caused controversy at Grand Slams.
Nadal now heads into the year's opening slam, where the top seed will kick off his campaign against Victor Estrella Burgos, with more questions than answers.
Caroline Wozniacki, the second seed after her resurgence in 2017 when she reached eight finals, opens against Mihaela Buzarnescu of Romania in the bottom half of the draw.
The Russian captured his first ATP title a year ago and reached the US Open quarter-finals - where he couldn't pull off a major upset and beat eventual champion Rafael Nadal.
Michigan State vs. Maryland
Kenny Goins took his place and buried the first two 3-pointers of his college career, a sign of things to come. That included holding Maryland scoreless for a 4:29 stretch in which the Spartans scored 15 straight points.
Meanwhile, 2017 Australian Open champion Serena Williams opted out of the competition, saying that her preparations had fallen just short of what she needed to be competitive after giving birth in September.
Federer took his Grand Slam tally to 19 after extended rest breaks and Nadal, who enters his 18th season on the ATP tour, has also benefitted from carefully picking tournaments to preserve his fitness.
Now ranked No47, she could face the 14th seed Anastasija Sevastova in the second round, Angelique Kerber - the 2016 champion - in the third round and, potentially, Garbine Muguruza, the Wimbledon champion, in the fourth round.
British number one Johanna Konta, seeded ninth for the first grand slam of the year, will take on Madison Brengle of the U.S. while last year's runner up Venus Williams has been drawn against Belinda Bencic of Switzerland.
Serena Williams will not defend the women's title having had her first child past year but 2008 champion Maria Sharapova, who failed a drugs test at the tournament in 2016, is back unseeded after a ban and will face Germany's Tatjana Maria. "You always have to beat the top players in order to get to the top".
"It was a fantastic test for me to see what we have done as a team in the off-season, whether it works or not on the court", said the Serb.
She said: "I have pain in my right adductor".