Though the match was incredibly close, thinks looked bleak for Cirstea as she served at 57 56, 15-40 - double match point down. But something must have clicked - the Romanian reeled off 15 points in a row from there, not only saving those match points and digging out of that game, but racing through the tie-break at love and holding serve at love in the first game of the third set on top of all of that.
Cirstea was in trouble again in the third, facing break point serving 4-4, 30-40, but she came alive again, winning seven points in a row to beat the No.9 seed in two hours and 56 minutes, 57 76(0) 64.
The Romanian's victory over the World No.10 was the milestone 10th Top 10 win of her career.
"I felt good. I felt solid. Obviously I made a few errors, but it's just my first match on hard in a really long time," Williams said after the match. "I also played a really good player, so it was a good match.
"It took me a few games to adjust to her game, but it's good that she played like that because of my next opponent. Now I feel like I'm completely adjusted, and I feel good going into the next round."
Next up is No.13 seed Kirsten Flipkens, who beat Dutch qualifier Kiki Bertens, 75 36 62. Flipkens had taken out Venus Williams in the first round this week - she has never played against Serena Williams.
Other winners later in the day were No.6 seed Petra Kvitova, who beat Canadian wildcard Eugenie Bouchard, 63 62; No.12 seed Samantha Stosur, who snapped a two-match losing streak against Carla Suárez Navarro with a 16 62 63 win; and No.16 seed Ana Ivanovic, who beat Flavia Pennetta, 64 64.
The Kvitova-Bouchard scoreline doesn't seem complicated, but Bouchard - the Canadian No.1 - pushed the defending Rogers Cup champion close in just about every game with her consistent power.
"I know I can play at that level and I know I can play better than I did today," Bouchard said in her presser. "Obviously I think she's a great player and I think she played very well today, so you have to give it to her. I think it was a hard-fought battle out there and tougher than the score would indicate."
The Canadian's confidence was evident when she was asked what experience she drew from it.
"I think it's another match, more experience," she said. "I have worked hard my whole life and I want to become a Top 10 player, so I need to play against the best. I always love the opportunity to play against a Top 10 player like Petra, so I was looking forward to playing. Even if I wasn't playing my best, I was fighting my best, and I gave myself my best chances. But she deserved to win tonight."