It was brief, fleeting, but it offers more than a clue to the psyche of the 20-year-old from Montreal who now stands on the brink of a breakthrough not only to a grand slam final but to a place in the celebrity world beyond the court.
Tough and talented, Bouchard, who will soon be confirmed as a top-10 player, had beaten the feisty Angelique Kerber of Germany 6-3, 6-4 and was asked when she first had the belief she was going to be a champion.
She replied without hesitation. The answer demands to be run in full. "Well, since I was young I've always been self-confident. I think it's something I had naturally. And also, maybe how I was raised. I've always believed in myself and was determined to do as well as I could in anything I did, no matter what it was, whether it was my homework or my tennis practice.
"I mean, when I was nine I decided to be a professional tennis player. So for me, professional tennis player is succeeding, top 20, top 10. You know, as I started playing more and more, I really had concrete dreams of winning a grand slam. Yeah, just going on, every time I play I realise, okay, I can play with this level and play with these top girls. Playing my first full year on the pro tour last year really showed me that, as well."
This, then, is a competitor who is not afraid to have ambition and not slow to realise it. She has now reached the semi-finals of every grand slam played this year.
She has not stumbled through a lack of belief. Wimbledon, too, does not disturb her. She is serene in her sense of belonging at the top of the game as befits one who has won both the singles and doubles title as a junior at the All England Club. She is extraordinarily focused, too.
There is a media storm gathering around Bouchard. She does little to discourage it, making smart choices on endorsements and having celebrity friends such as Jim Parsons who stars in The Big Bang Theory.
But she is dedicated to performing with a racket in her hand. Asked about the frenzy over her Wimbledon campaign in her native Canada, she said: "I don't have a huge sense of it because I'm across an ocean and in my own kind of bubble. I'm not really reading anything or caring too much about the outside, you know, talk."
The inside, you know, talk is all about her meeting today with Simona Halep of Romania. The other semi-final will be contested by Lucie Safarova and Petra Kvitova in a Czech Republic derby match. Kvitova leads this rivalry 5-0 and will be expected to prevail again.
Bouchard v Halep is harder to call. They have played only once when Halep won in three sets in Indian Wells. "I learned from that match that I have to keep fighting till the end. At that moment I won, so we'll see tomorrow what will happen," said the world No.3.
It promises to be one of the highlights of a tournament that has provided more fireworks than the Chinese New Year.
There is a contrast in their personalities. Halep is shy, even reserved. But this reticence should not be confused with weakness. The 22-year-old yesterday notched her 250th career win.
She may be quiet but she has faith in her ability. "I believe that I can win, but I expect a tough match," she of meeting Bouchard. "She's a great player and she's playing really well, so I have just to enjoy it."
She was also asked if she could handle the stress on a Centre Court that has witnessed the occasional meltdown from a player.
"Yes, I can handle the pressure," she said. "I don't feel now pressure because I have experience from Australian Open also quarter-finals and the French Open finals. I feel very relaxed now. I'm focused. I just want to enjoy every match."
The beaten opponents of both Halep and Bouchard pointed to a similarities shared by their conquerors. They told of the confidence of both and their accuracy of shot in the most crucial of moments.
The statistics point to another truth. Halep and Bouchard have each won 15 grand slam matches this season, more than any other player. The fight to see who notches up No.16 will enthral Centre Court today.