Bouchard, who came from 75 53 down to beat No.9 seed Sloane Stephens in the second round, won nine of 11 games from 5-4 down in the first set to beat No.6 seed Jankovic in the third round, 75 62.
The Canadian's fortunes changed quite clearly after that 5-4 changeover. Perhaps her coach, Nathalie Tauziat, had something to do with it - the former World No.3 came out for some on-court coaching.
"I wasn't at my best in the first set - she was controlling the points more, and I wasn't aggressive enough," Bouchard said. "Nathalie told me to be more aggressive, which is my game. So I started to do that, and it worked out. I turned the match around from that point on, so it was helpful advice."
This is Bouchard's first time playing the Toray Pan Pacific Open, though it's not her first time being at this tournament - her quarterfinal run this year might help bury her disappointment from last year.
"Last year I came here and thought I would get in, but I didn't even get into qualies - I was two out of qualies. So I was just here practicing. But this year my luck changed because I was last in the main draw. Of course my ranking's improved in the past year, so getting in this year is thanks to that.
"Now I'm just taking advantage of being main draw. I'm happy to play as many matches as I can."
Bouchard's win over the No.10-ranked Jankovic is her second Top 10 win, after beating a No.9-ranked Sam Stosur at Charleston earlier this year (though Stosur had to retire in that match with a calf injury).
"I thought in general I played really well," Wozniacki said after the match. "She's a tough opponent because she's mixing up the game quite a bit, but I didn't let that hinder me, and I was just playing my game and putting the pressure on her out there. I thought I played really steady in the match today."
After winning the first set Halep was up a break three times in the second set and even led 2-0 in the third with a point for 3-0, but Williams never stopped fighting, winning six of the last seven games of the match - and 12 of the last 13 points - to close it out and move on through to the quarterfinals.
"It was very tough - she played so well, she played amazing," Williams said in her on-court interview. "I don't know how I was able to win today - I just wanted to stay in Japan longer and play for you guys!
"But I'm very happy to arrive in the quarterfinals. I'm taking it one match at a time and I like it."