The No.1-seeded Williams, who had won all 20 sets she played since Wimbledon, lost serve in the first game of the match and never looked comfortable en route to losing the first set in 46 minutes, 6-4.
"I was just really - every ball I hit today seemed to be going out," Williams said about the first set after the match. "I even had the easiest approach shot and it didn't just go slightly wide, it was like 50 feet out. That was actually embarassing. I thought I'd never hit a shot like that professionally. I have maybe in practice and with my eyes closed, but I've never hit a shot like that when I'm conscious.
"It was just some crazy tennis out there today. It was just really different conditions than last week in Toronto - I always forget it's so different here. So it probably took me a while to get used to it."
Williams is never one to back down from a challenge though - in the first game of the second set she fired three aces and held easily. She raced out to a 4-1 lead in the second set and 4-0 in the third, and she never really looked back in either set to cruise past the Canadian qualifier in three, 46 62 62.
"She has a good game," Williams said of Bouchard. "She has a good serve. She moves really well. She hits the ball really, really well. There's a reason she won Wimbledon juniors. There's a reason she did well at Wimbledon this year. She's flying up the rankings, and it's because she's really good."
But there's still work to do - since this tournament was upgraded to a Premier-level tournament in 2009, Williams has never been past the quarterfinals, and she spoke afterwards about changing that.
"Apparently I've done well here once - I got to the semifinals, but it was a smaller tournament back then," she said. "I have to get used to these conditions because this is a tournament I don't have.
"If I play like today, I'll continue not to have it."
Barthel isn't exactly intimidated by World No.1s, either - she had a pair of near-wins over Victoria Azarenka when she was ranked No.1 last year, pushing her to a 64 67(7) 76(6) scoreline at Indian Wells (she led 4-1 in the third set) and 64 67(7) 75 at Stuttgart (she was up a break in the third set).
Azarenka voiced her impression after the Indian Wells match: "She played some unbelievable tennis. I never really heard much about her before this year. She's definitely coming up, and very dangerous."
And after the Stuttgart match: "Mona has been showing some excellent tennis throughout the year. She has a big serve and big strokes, and she's just coming up, so she has absolutely nothing to lose."