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Serena Williams, Naomi Osaka headline Day 2 at the Australian Open

MELBOURNE, Australia -- Opening day of the Australian Open has come to a close, and for the most part, everything played out as we thought it would.

Except for John Isner, the No. 9 seed, that is. He lost to a fellow American, 6-foot-11 Reilly Opelka, becoming the only top-10 player in the tournament to fall on Day 1.

But defending champs Roger Federer and Caroline Wozniacki put on first-round masterclasses in the evening session. How will things play out on Day 2? Let's have a look at some of the highlights:

How to watch

Day session

ESPN3 & ESPN Plus: Starting at 7 p.m. ET

ESPN2: Starting at 9 p.m. ET

Night session

ESPN2: Starting at 3 a.m. ET

*Full Day 2 schedule here

Key storylines

Serena and Osaka to kick off their 2019 Grand Slam seasons

Four long months after a memorable US Open final, Serena Williams and Naomi Osaka will take the court.

Williams, seeded 16th in Melbourne, is second on Rod Laver Arena on Tuesday. She takes on German Tatjana Maria, while later on the same court, No. 4 Osaka faces Magda Linette of Poland.

While she hasn't played an official match in 2018, Williams looked sharp at Hopman Cup, winning three singles matches.

Williams and Osaka could meet in the semifinals.

An uphill battle for 16th seed Raonic

It goes without saying that it pays to be a seeded player at a Slam, but Milos Raonic might not feel that way after drawing Nick Kyrgios in the opening round.

Raonic no doubt would have preferred a more favorable path, but with an opening match against the two-time Grand Slam quarterfinalist Kyrgios, it isn't looking like the easiest start.

The kicker is that should Raonic (or Kyrgios, for that matter) win, it'll only get more difficult, with 2014 Australian Open winner Stan Wawrinka looming as a second-round matchup.

Can lightning strike twice for Kanepi against Halep?

It's not often you'll find a No. 1 seed dreading a first-round match, but Simona Halep might be more than a little nervous heading into her opener against No. 71 Kaia Kanepi.

Why? Halep was eliminated in the first round of the 2018 US Open by Kanepi.

Kanepi isn't a stranger to knocking out top-10 players. She's done it 11 times in her career. And to make matter worse, Halep hasn't been 100 percent healthwise, either.