The Montreal native surprised herself by finishing second in the semifinals, a result she wouldn't have predicted ahead of her first dive.
"This morning my training was awful so I didn't know what to expect," said Benfeito.
"I know I made some mistakes this morning so I can still improve my dives for the final."
The Montreal native had 359.90 points to put her in medal contention, but still well back of defending champion Chen Ruolin of China ahead of the final later Thursday.
Chen was one of only two divers to score more than 80 points with a single dive — but she did it four times — to finish with a total of 407.25 points.
"It was so-so," Chen said. "There were some dives that were not perfect, some entries that were not perfect."
Brittany Broben of Australia was third, less than half a point behind Benfeito with 359.55.
Roseline Filion of Laval, Que., also qualified for the final after finishing eighth with 329.60 points. The top 12 finishers advanced.
"Today we saw that there is no big difference in the results so everybody can get to the podium," said Filion. "Tonight I can still improve with the adrenaline and I will just enjoy myself."
Melissa Wu of Australia, who earned a silver in 10-meter synchro four years ago, qualified fourth, while Pandelela Rinong Pamg of Malaysia was fifth.
Wu wasn't ready to hand Chen victory.
"Gold is never sewn up," she said. "I just want to do five good dives and that's my goal for the final."
Pamg, who finished second to Chen in Wednesday's preliminary round, competed with a swimsuit featuring an image of the Olympic torch, with the word "Dream" written below the flame.
The top 12 finishers advanced to the final Thursday evening.
China has won five of six diving golds so far in London, including the women's 10-metre synchronized title claimed by Chen and Wang Hao. The country's attempt to sweep all eight golds was spoiled when Ilya Zakharov of Russia won the men's three-metre springboard on Tuesday.
"It's encouraging me more," Chen said. "I'm taking my competition even more seriously."
Chen stood second after the first round, which was led by Yulia Koltunova of Russia, who executed a difficult armstand dive well. However, Koltunova then fell back and qualified sixth.
The only other diver to break the 80-point barrier was Broben, who earned 81.60 points for her second dive, an inward 3 1/2 somersault tuck.
Chen's most successful dive came in the last round, when she was awarded 84.80 for a back 2 1/2 somersault pike with 1 1/2 twists, which drew loud cheers from the morning crowd inside the Olympic Aquatics Centre.
Chen won both the individual and synchro titles before her home fans at the 2008 Beijing Games as a 15-year-old. She then swept both events at last year's world championships in Shanghai.
Still, it wasn't a completely successful morning for the favourites. China's other competitor, 16-year-old Hu Yadan, qualified just ninth. Hu took silver behind Chen at last year's worlds.
Christin Steuer of Germany advanced in seventh. She was in London as a reserve athlete and only got into the competition when her teammate Nora Subshinski withdrew because of a neck injury.
Paola Espinosa Sanchez of Mexico, who took silver with Alejandra Orozco in 10-metre synchro last week and bronze in the individual event at last year's worlds, just made it in 11th.
— With files from the Associated Press.
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