Stephanie Brantz and Cliff Drysdale look ahead to day 6 of the Australian Open. (2:25)

MELBOURNE, Australia -- The third round of the Australian Open continues Saturday and is highlighted by the return of world No. 1s Novak Djokovic and Simona Halep, both of whom will be hoping for a spot in the second week.

It's also a big day for the Americans. Serena Williams, Venus Williams and Madison Keys are all in action.

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 6 schedule here

Key storylines

Serena leads the American women's charge

Serena Williams of the United States returns a ball during her second-round match at the 2019 Australian Open. Jason Heidrich/Icon Sportswire via Getty Images

The Williams sisters and Keys will be looking to join compatriots Sloane Stephens and Danielle Collins in the fourth round when they take to the courts Saturday.

Serena is second up on Rod Laver Arena and faces Ukraine's Dayana Yastremska, while Keys goes up against 12th seed Elise Mertens next door at Margaret Court Arena.

But Venus will have the sternest test of all, a date with last year's runner-up and top-ranked Halep. Williams, 38, has the lifetime 3-2 edge in head-to-heads, and based on what we've seen from both so far in Melbourne, this has all the makings of another tight and gripping contest.

First-time test for Djokovic

Novak Djokovic of Serbia reacts after winning a point against Jo-Wilfried Tsonga at the 2019 Australian Open. JEWEL SAMAD/AFP/Getty Images

Six-time Australian Open champion Novak Djokovic hasn't needed to shift from first gear so far this tournament. He effortlessly took out American Mitchell Krueger in straight sets before seeing off former Melbourne Park finalist Jo-Wilfried Tsonga in an equally one-sided contest.

Next up for the Serbian is rising start Denis Shapovalov, the 25th seed, who like Djokovic hasn't dropped a set en route to the third round.

These two have never faced each other. Could an unfamiliar foe spell danger for Djokovic?

Nishikori returns after back-to-back five setters

Kei Nishikori of Japan celebrates after defeating Ivo Karlovic in the second round of the 2019 Australian Open. NaFoto/Action Plus via Getty Images

It took Kei Nishikori 10 sets of tennis to bring up his first match point of the tournament. His first-round opponent, Kamil Majchrzak, was forced to retire injured midway through their fifth set, and things didn't get much easier in the second round, with Ivo Karlovic sending down 67 aces in a five-set slugfest.

Nishikori might be through to the third round, but he has already spent 6 hours, 36 minutes on court. Only one man has played more tennis through the opening two matches -- Nishikori's next opponent, Joao Sousa. The Portuguese journeyman has tallied a whopping 8 hours, 19 minutes so far, with both of his matches passing the four-hour mark.

The legs will be feeling heavy for both Nishikori and Sousa. At least a cooler day in Melbourne is likely to bring some relief to their scheduled afternoon start time.