Paula Badosa advanced to her first Grand Slam semi-final after beating Coco Gauff.
Osaka made her own comeback 12 months ago after a maternity break during which she had a daughter.
Following a marathon 90 minutes of play, it was Prannoy who was left to bear the heartache as the 9-21 23-21 20-22 loss against world number 24 Weng robbed him of a chance to win his second BWF title of the season.
The world number nine Prannoy took 43 minutes to ward off 21-year-old Rajawat's challenge 21-18 21-12.
The Brazilian pair of Luisa Stefani and Rafael Matos beat India's Sania Mirza and Rohan Bopanna 7-6 (2), 6-2 to win the Australian Open mixed doubles title.
The 22-times Grand Slam champion appeared to suffer a strain in his left hip area while running for a backhand in the second set at Rod Laver Arena and after attention from a trainer, took an off-court medical time-out when trailing 6-4 5-3 at Rod Laver Arena.
The 42-year-old Williams has reached the final twice in Australia, including a loss to her younger sister, Serena, in the 2017 championship match.
The 35-year-old Spanish legend, seeded sixth, was too solid for the Italian seventh seed winning 6-3, 6-2. 3-6, 6-3 in 2hr 55min and will face either Daniil Medvedev or Stefanos Tsitsipas in Sunday's final.
The world number one player left Australia late on Sunday after the Federal Court upheld a government decision to cancel his visa, capping days of drama over the country's COVID-19 entry rules and his unvaccinated status.
"Innovation is one of the hallmarks of the Australian Open, and alongside Infosys we are constantly striving to take tennis to a new level in terms of delivering insightful and engaging digital content," Australian Open Tournament Director Craig Tiley was quoted as saying in an Infosys statement.