Osaka advanced to face 16th seed Clara Tauson, who eliminated sixth-seeded Madison Keys 6-1 6-4 in the first quarter-final on Tuesday night.
"Not my best performance out there today," said Keys, who won this year's Australian Open.
"She played incredibly well, she served really well. It was just one of those things where I felt like I kept trying different things, and she was just beating me."
Osaka, the four-time grand slam champion and former world No.1, continued her best performance in a WTA 1000 tournament since reaching the final in Miami in 2022. She stepped away from tennis for 15 months towards the end of that season and had daughter Shai in July 2023.
The native of Japan is seeking her eighth career title and her first since the 2021 Australian Open.
Canadian teenager Victoria Mboko takes on ninth seed Elena Rybakina of Kazakhstan in the other semi on Wednesday.
Mboko, the 18-year-old from Toronto who's playing in her first main draw in Montreal, has defeated five higher-ranked opponents en route to the last four, including top seed Coco Gauff.
Tauson dedicated her victory to her grandfather, Peter, during an on-court interview at IGA Stadium.
"I really wanted to win for him today," she told the crowd before breaking down in tears.
"I really wanted to come out here and show my best tennis for him, and hopefully he's watching."
Tauson, who is yet to drop a set all tournament, said she only found out about her grandfather's death on Monday, a day after she eliminated Wimbledon champion and No.3-ranked Iga Swiatek.
"He used to coach me a little bit playing tennis and drove me to almost every single practice from my school," the 22-year-old native of Denmark said.
"It was tough news yesterday morning when I woke up, but I think it's for the better. He was not feeling great for a while."
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