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.
Czech Petra Kvitova jumped two spots to eighth in the latest WTA rankings released Monday after beating Kiki Bertens to win the Madrid Open for the third time on the weekend.
Denis Shapovalov, regarded as one of the brightest stars on the ATP Tour and ranked at 43 in the world, fired 29 winners past Kyle Edmund.
Simona Halep improved to 16-1 on the season and said her confidence is what keeps her going in tight matches.
US ninth seed Williams, seeking her third US Open crown, advanced to a Thursday matchup for a spot in Saturday's final against 83rd-ranked compatriot Sloane Stephens.
Maria Sharapova is expected to jump from 146th to around 100th in the world rankings.
Karolina Pliskova became the firm Wimbledon favourite on Wednesday when compatriot and two-time champion Petra Kvitova was knocked out.
Venus Williams wept at Wimbledon over the "devastation" of a fatal road accident for which she is facing legal action as Andy Murray and Rafael Nadal eased into the second round on Monday.
With the 23-time Major winner preparing for the birth of her first child and the Russian out with an injury, the race to the Venus Rosewater Dish is wide open.
The two-time Wimbledon champion swept aside American Julia Boserup 6-3, 6-2 in her first match of the French Open.
Petra Kvitova will make a "last-minute decision" whether to compete in the French Open as she prepares a comeback five months after her playing hand was badly injured in a knife attack.
The twice Wimbledon champion's left hand was badly injured in a knife attack at her home in December last year.