MELBOURNE, Australia -- It's still only the second round, but the action is starting to heat up at the Australian Open.

Novak Djokovic and Simona Halep, both No. 1 seeds, have difficult tests on Rod Laver Arena, while Serena Williams and Eugenie Bouchard will need to set aside their mutual admiration in their second-round match.

Bouchard, while trying to avoid "putting [Williams] on too much of a pedestal," said she "loves" Serena, while Williams was equally as impressed by the Canadian's recent "fight" to find her form.

Hold on to your hats (and put on some sunscreen): It's Day 4 of the Australian Open.

How to watch

Day session

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

ESPN2: Starting at 9 p.m. ET

Night session

ESPN2: Starting at 3 a.m. ET

*Full Day 4 schedule here

Day 4 matches here

Key storylines

Serena faces tough challenge in former Wimbledon runner-up

Seven-time Australian Open champion Serena Williams had no issues during her opening match. Mark Kolbe/Getty Images

Serena Williams hasn't played many former Grand Slam finalists in the second round of majors, but she will Thursday. Her opponent, Eugenie Bouchard, is starting to show signs of the level we saw during her breakout year in 2014.

It's been nearly five years since Bouchard reached the Wimbledon final, and this season, although early, she is reminding us of how talented she is. In Auckland, she reached the quarterfinals in the singles draw and won the doubles title with partner Sofia Kenin.

Bouchard has a 13-5 record at the Australian Open, her best winning percentage at any Slam.

Raonic, Wawrinka face off in hard-hitting second-round clash

Canada's Milos Raonic and Switzerland's Stan Wawrinka at the 2018 US Open. EDUARDO MUNOZ ALVAREZ/AFP/Getty Images

Two nights after taking out Nick Kyrgios in the first round, 16th seed Milos Raonic isn't getting a break. He will be squaring off against three-time Slam winner Stan Wawrinka.

Ranked as low as 263 in the world last June after battling a series of chronic knee issues in 2017 and 2018, Wawrinka started to show great form in the back half of the 2018 season.

Currently ranked No. 53, the 2014 Australian Open winner may be a handful for Raonic, though the Canadian can take solace in his straight-sets win over Wawrinka at last year's US Open.

No. 1 seeds to face stiff early tests

Top-seeded Simona Halep faced a tough challenge in the first round, and she's trying to keep her No. 1 ranking in Melbourne. WILLIAM WEST/AFP/Getty Images

She narrowly avoided a first-round setback, but the first week of the Open isn't getting easier for No. 1 seed Simona Halep. After already staving off a firm challenge from Kaia Kanepi, Halep faces up-and-coming American Sofia Kenin on Thursday.

Kenin, 20, has improved from 57th in the world to 37th. Earlier this year, she won her maiden WTA trophy, in Hobart, defeating Caroline Garcia, Kirsten Flipkens and Alize Cornet along the way.

In a rematch of the 2008 final, men's No. 1 seed Novak Djokovic faces Jo-Wilfried Tsonga. The Frenchman has not fallen before the third round since 2007. But then again, Djokovic is a six-time Australian Open winner.