Bouchard, ranked seventh in the world, lost 6-3, 6-1 on Wednesday, a thorough beating that left her bothered and dissecting every aspect of her game.
"As soon as it was over, definitely a bit of anger, but also kind of this confusion, slash, like quest to find what's wrong, like this kind of searching feeling that, 'OK, like I know something's off,'" Bouchard said. "I know something's not right. So I want to find it."
A year ago, everything was going right for Bouchard. She reached the semifinals here, starting a run where she advanced to the French Open semifinals and Wimbledon final.
Bouchard has not played like that this year, losing her opening tournament match for a second straight event after falling to Tatjana Maria at the Miami Open.
Davis, a 21-year-old American, was the aggressor throughout, something Bouchard didn't fully understand.
"I was definitely a little bit slow today, overpowered," Bouchard said. "Usually, I'm the one dominating. So it was definitely, just, I don't know, just not good."
Davis had lost the only career meeting with Bouchard, but was in control much of the way against the world's seventh-ranked player. Davis won nine of the final 11 games of the match and broke Bouchard's serve twice in the final set to pull off the upset.
"I've never beaten here before, so I came out knowing I had nothing to lose," said Davis, ranked 66th in the world.
Bouchard was a semifinalist last year and, along with No. 2 seed Ekaterina Makarova, one of only two top-10 players in the field. She wore a black wrap around her right ankle.
Davis used precise ground strokes to keep Bouchard on her heels throughout the second set. Davis broke Bouchard's serve at love to take a 3-1 lead as the 21-year-old Canadian had a double fault, then sailed a forehand long to fall behind.
Two games later, Davis hit a backhand to send back Bouchard's 102 mph serve for a winner down the left sideline and a 5-1 lead. The match ended on Bouchard's forehand into the met, Davis waving to the cheering crowd.
"I was pretty calm and relaxed about it," Davis said. "I was really happy inside."
Bouchard's loss came a short time after Makarova won her opening match over Zhang Shuai 5-7, 6-3, 6-3, then withdrew because of a gastrointestinal illness.
Makarova felt stomach pain during her match with Zhang and saw trainers after the match. When things didn't get much better, she said she and her team decided it was best to rest and get ready for the remainder of the clay-court season.
Her departure means 13th-seeded Irina-Camelia Begu, the 26-year-old Russian's next opponent, will advance to Friday's quarterfinals.
"I'm really disappointed to have to withdraw here in Charleston," Makarova said in a statement. "It's one of my favourite tournaments and I've really enjoyed my time here these past few days. Unfortunately, I felt pain in my stomach today during the match and called the trainers, but it hasn't got any better."
Makarova overcame a slow start to get past Zhang. Makarova, who lost in the doubles finals at the Miami Open on Sunday, said she hadn't gotten to practice on clay before arriving at the Family Circle Tennis Center.
She played through the discomfort to win, yet thought she needed to rest and not let the illness linger.
Begu, of Romania, advanced by defeating Kazakhstan's Yaroslava Shvedova, 6-0, 6-7 (4), 6-4.
In other matches, No. 4 seed Sara Errani of Italy topped last year's finalist, Jana Cepelova of Slovakia, 6-3, 7-6 (5).
Seventh-seeded Madison Keys became the third player from the United States in the round of 16 when she beat Kateryna Bondarenko of Ukraine, 6-2, 6-1.
Past Family Circle champion and former U.S. Open winner Sam Stosur of Australia, the ninth seed, fell in three sets to Lara Arruabarrena of Spain, 6-3, 3-6, 6-4. No. 15 seed Mona Barthel beat American Sloane Stephens 6-3, 7-6 (2). No. 12 seed Belinda Bencic of Switzerland lost to Danka Kovinic of Montenegro, 4-6, 6-3, 6-2.