It was the third straight upset for the seven-time Grand Slam champion. Next, she faces No. 5 seed Petra Kvitova in the semifinals.
The way Williams is winning is as impressive as the victories themselves.
Williams, 33, is defeating far younger players in long, grueling matches. She defeated No. 13 seed Simona Halep, 21, in a three-setter the day before. Williams' win over Bouchard, 19, took three hours and two minutes.
In the first round, Williams upset No. 1 seed Victoria Azarenka, 24, in straight sets.
Four years ago, Williams reaching the semifinals was no big deal. But since disclosing that she suffers from Sjogren's Syndrome in 2011, Williams has struggled against higher-ranked opponents and in long matches.
She plummeted outside of the top 100 in 2011. She fought to climb back into the top 25 in 2012 but fell out of the top 50 this year because of nagging back issues. She is currently ranked No. 63.
With the exception of a good showing in Charleston, Williams' play had become predictable. She would show flashes of brilliance but then sputter and finally tank.
She raced out to a 6-0 lead against Kirsten Flipkens in Toronto, but Flipkens bounced back and beat her 0-6, 6-4, 6-2.
She refused to listen to postmatch retirement talk.
It was as if Williams knew hanging tough would eventually pay off.
What Williams needed was court time. She has played in only 10 tournaments and fewer than 25 matches this year.
She played doubles with sister Serena Williams at the U.S. Open. It was the first time the two had played doubles since the Australian Open.
Doubles offered Williams valuable time on the court. The further they went in the tournament, the better Williams played. They upset the No. 1 doubles team of Sara Errani and Roberta Vinci but lost in the semifinals to eventual winners Andrea Hlavackova and Lucie Hradecka.
More matches will help Venus regain consistency and confidence—things she clearly lacked most of this year.
However, missing so many tournaments in 2013 actually improves her chances next year. She will have fewer points to defend. Every match won will be a points bonanza.
Venus has said she wants to play doubles at the 2016 Olympics in Brazil.
If she keeps playing like she has played in Tokyo, she may even be able to play singles.