Ashleigh Barty beat American 18th seed Alison Riske 6-3, 1-6, 6-4 and next plays two-time Wimbledon champion Petra Kvitova.
Petra Kvitova lost the first set before winning 6-7 (4/7), 6-3, 6-2 to ruin Maria Sakkari's hopes of becoming the first Greek woman to reach the last eight of a Grand Slam.
Ashleigh Barty and Belinda Bencic both advanced to the semi-finals after contrasting victories at the WTA Finals in China.
Naomi Osaka maintained her unbeaten record over Petra Kvitova having won their only previous clash in a three-set classic in Melbourne.
Roger Federer admitted he struggled to get past the talented Gael Monfils.
Petra Kvitova, chasing a fourth title on the Caja Magica clay, has now won her last five matches over Kristina Mladenovic in straight sets.
Venus Williams rallied from a set and two breaks down to topple third-seeded Petra Kvitova 4-6, 7-5, 6-4 at Indian Wells.
Japan's Naomi Osaka retained the world number one spot.
Naomi Osaka said she was not satisfied with back-to-back majors Sunday and hoped to complete a "Naomi Slam".
Comeback queen Petra Kvitova said she was hurting but still felt like a winner.
Naomi Osaka battled past eighth-seed Petra Kvitova 7-6 (7/2), 5-7, 6-4 to win her first Australian Open crown.
Australia Open 2019 Women's Final, Highlights: Naomi Osaka defeated Czech Republic's Petra Kvitova 7-6 (2), 5-7, 6-4.
Naomi Osaka will face Petra Kvitova for the women's singles Australian Open title.
The fairytale runs of Stefanos Tsitsipas and Danielle Collins were also extended as they too swept into the last four.
Australian Open 2019: Ashleigh Barty stunned five-time Grand Slam winner Maria Sharapova in a three-set thriller.
Serena Williams won her 23rd title and seventh at Melbourne Park while eight weeks pregnant two years ago.
Former world number one Karolina Pliskova booked her spot in the semi-finals for the second straight year having also beaten Wozniacki earlier in the tournament.
Kvitova has now lost both her matches and needs results to go her way in the White Group to stay afloat in the season-finale.