The Australian, whose matches have been 5,000-strong sellouts all week at Memorial Drive, will face Dayana Yastremska on Saturday for the title.
The 19-year-old Yastremska, ranked 24th, will play either Australian world number one Ashleigh Barty or American Danielle Collins.
The Spaniard is attempting to match Roger Federer's record 20 Grand Slam singles titles on his least favourite surface, having won in Melbourne just once, back in 2009.
The 22-year-old Japanese had a rollercoaster 2019, winning in Melbourne and topping the world rankings but then ploughing through three coaches before settling on Wim Fissette in December.
The reigning Wimbledon champion and former world number one dropped serve five times and was flattened in the second set by her opponent from Belarus.
There was only one break in each set of the evenly contested clash with Humbert winning 7-5, 6-4, clinching the victory with the Canadian's sixth double fault.
The world number 21 and Australia's top player pulled out of the Adelaide International with the same problem and has not been able to recover for his home Grand Slam.
The WTA Premier 5 tournament features eight of the world’s top-ten players.
The two tennis legends were at the Rod Laver Arena in a bid to swell the vast sums already donated to help victims of the devastating disaster.
The former number one, who celebrated on court with daughter Olympia, donated her US$43,000 winner's cheque to victims of the Australian bushfires.
A medical evaluation on court ended with the 31-year-old world number 18 calling it quits against her teenaged opponent.
In Melbourne, a picturesque bayside city famed as one of the most liveable in the world, the bushfire smoke raised pollution levels to "hazardous" at the start of the week.
The revelation came after another South American player, Chile's Nicolas Jarry, was provisionally suspended by the ITF after testing positive for banned substances at the Davis Cup in November
The Australian was joined in the last eight by second seed Simona Halep, who made her 2020 competitive debut with a 6-4, 7-5 defeat of Ajla Tomljanovic.
Mirza, regarded as her country's greatest women's tennis player, became the first Indian to win a WTA singles title in 2005.