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.
Sharapova, the 2004 champion, was joined at the exit by 2011 and 2014 winner Petra Kvitova who was stunned by Belarusian world number 50 Aliaksandra Sasnovich.
Serena, 36, missed Wimbledon last year while preparing to give birth and has endured a rocky time since resuming her career earlier this year.
Halep, the runner-up in Paris in 2014 and 2017, slumped 0-5 down in the opening set to world number 83 Riske whose only win at the tournament came in 2014.
Petra Kvitova is one of the favourites to win a maiden French Open title after winning four titles this season, including on clay in Prague and Madrid.