The No.6-seeded Radwanska was in complete control early on - with her pinpoint groundstrokes hitting their mark and her trick shots in full flow, she reeled off the first seven games to lead, 6-0, 1-0.
But the No.2-seeded Halep came alive from there, finding the range again on her consistent yet penetrating ground game to go up 3-1 in that second set, and after Radwanska caught back up to 3-all, it was all Halep - the Romanian won nine of the next 10 games to run away with it, 0-6, 6-3, 6-1.
"At the beginning I couldn't move my legs. I just felt very tired. And she was playing amazing - I couldn't do anything," Halep said. "But I just kept focusing on what I had to do on the court - I knew I had to change my tactics, so I just kept the ball inside the court more and made more rallies out there.
"The fans kept cheering for me even when I was down 5-0, and it helped me keep fighting."
Halep, who had never even been past the second round of the Rogers Cup beforehand, is now through to the semifinals of the Premier 5-level tournament for the first time in her blossoming career.
The World No.3's win over Radwanska was also the milestone 300th match win of her career.
She was asked if she remembered the first of those 300 wins.
"No, I don't remember, but for sure it was very important for me," she said.
And the most important of those 300 wins?
"Serena, last year in Singapore, is the best win for me," she said. "But every match is important at this level because you play with top players and you always have a big challenge when you go on court.
"So every day, and every victory, is important. And I want more. That's why I'm preparing every day. I'm working hard. Of course I won't give up now - I'm just looking forward to the next tournaments."
Awaiting Halep in the semifinals will be No.15 seed Sara Errani, who ended the run of Ukrainian qualifier Lesia Tsurenko, 6-4, 6-4. Halep leads Errani in the pair's head-to-head series, 2-1.
"Errani is running very well on the court, and it's going to be another tough one," Halep commented.
"I just hope I have a better plan and can start the match better than I did today."