Rafael Nadal was the top seen on the men's side, while Karolina Pliskova was on the top in the women's tournament.
It should be an unpredictable tournament this year, as past champions Andy Murray, Novak Djokovic, Stan Wawrinka and Sam Stosur were forded to withdraw due to injuries. Meanwhile, perennial favorite Serena Williams is out on maternity leave.
And the unpredictability was on display on the first day of action. Maria Sharapova, who is in the tournament thanks to a wild card berth, shocked the second-seeded Simona Helep in three sets.
“Despite not playing a lot of matches coming into this, it almost seemed like I had no right to win this match today,” Sharapova said. “And I somehow did. I think that is what I'm most proud of.
Johanna Konta, who although seeded seventh was expected to be an Open contender, was bounced in the first round by Serbian Aleksandra Krunic in three sets. It was the first win in a major for Krunic, who is ranked 77th in the world, since Wimbledon in 2015.
With so many men pulling out of the tournament, suddenly the fourth-seeded Alexander “Sascha” Zverev becomes a potential favorite to win the Open.
But he didn't look great in his first-round matchup against Darien King of Barbados. The German took care of him in three sets, but it was far from a definitive victory.
“I obviously didn't play my best, but I thought I was hitting the ball well,” said Zverev. “He always had answers to my quick balls, to my pace. He's a very, very quick player.”
Elsewhere on the men's side, Jordan Thompson defeated American and 13th seed Jack Sock in five sets, while fellow American John Isner advanced to the second round.
(Photos: Michael O'Kane)