No.2 seed Angelique Kerber faces fellow left-hander Beatriz Haddad Maia for the first time. Victory for the German would seal her 100th Grand Slam main draw win - the eighth active player to achieve this. 22-year-old qualifier Haddad Maia, the former World No.58, is returning from a back injury that sidelined her last summer, and is seeking to become the first Brazilian player to reach the third round in Melbourne in the Open Era.
No.5 seed Sloane Stephens takes on a familiar face in Timea Babos today. The 25-year-olds were doubles partners in the junior ranks, and put together a 14-match winning streak in 2010 to capture the Roland Garros, Wimbledon and US Open doubles titles. At pro level, though, they paired up in just two tournaments in 2012 and lost both of their matches. Stephens and Babos have only met across the court once - a 6-2, 3-6, 6-3 win for the American in the second round of Strasbourg in 2012.
Timea Babos has just one Top 10 win in 21 matches against opponents in that echelon - and it came almost exactly a year ago when the Hungarian ousted CoCo Vandeweghe from the Australian Open in the first round 7-6(4), 6-2.
Belinda Bencic's only previous completed match against Yulia Putintseva was the Swiss player's first ever appearance at WTA level - in Luxembourg qualifying in 2011 when she was just 14 years old. Putintseva won that encounter 6-1, 1-6, 6-2, as well as a victory via retirement in the first round of Sydney in 2017.
Defending champion Caroline Wozniacki takes on Johanna Larsson for the first time. The Swede's overall record against Top 10 players is 2-18, and a victory over Wozniacki would be her first ever Top 5 win.
No.20 seed Anett Kontaveit leads a tight head-to-head against Aliaksandra Sasnovich 4-3, including their most recent encounter in Madrid last year. However, on outdoor hard courts, the in-form Sasnovich - who reached the quarterfinals in Brisbane and semifinals in Sydney - leads the series 2-0, having defeated Kontaveit in both Brisbane and Indian Wells last year.
Petra Kvitova leads the head-to-head against Irina-Camelia Begu 3-0, dropping only one set across those matches - in their most recent encounter last year in St Petersburg, which the Czech won 6-3, 1-6, 6-1. This will be their first ever meeting on outdoor hard courts.
2008 champion Maria Sharapova's 6-0, 6-0 first-round defeat of Harriet Dart was the eighth double bagel win of the 31-year-old's career, and first since her third-round shutout of Paula Ormaechea at Roland Garros 2014. Her opponent today, Rebecca Peterson, will be seeking to upset a Russian seed to make the third round at a second consecutive major, having beaten Anastasia Pavlyuchenkova to reach that stage at the US Open.
Sachia Vickery and Danielle Collins will take a long-standing rivalry honed in the minor leagues to the big stage today. All five of their previous meetings have taken place in North American ITF or 125K events, with Vickery edging the current head-to-head 3-2 - including their most recent bout 6-4, 7-6(2) in the first round of the Honolulu 125K at the end of 2017.
No.9 seed Kiki Bertens leads her head-to-head against Anastasia Pavlyuchenkova 3-1, though they are 1-1 on outdoor hard courts and the Russian won their most recent tilt in the second round of Wuhan three months ago 6-4, 6-2. Pavlyuchenkova is yet to crack the Top 10 herself - but has notched up 28 Top 10 wins so far in her career.
Former doubles World No.1 Bethanie Mattek-Sands has spent the past year making her comeback from a knee injury at Wimbledon in 2017 so horrific that it went viral. In a new piece for Players Voice, Mattek-Sands tells her story in her own words - and why she wants to be "remembered for her individuality".
The first rounds of majors can be even more significant for mid-ranked players as those at the top - and, focusing on milestone wins for Danielle Collins and Katie Boulter, Matt Zemek breaks down why "how the other half live" has so much importance in the sport for Tennis With An Accent.