The American sixth seed won nine games in a row as she came back from 4-3 down in the first set to oust the Chinese in just 51 minutes.
Keys, only 18, felt her serve helped her into a fourth career WTA quarter-final.
"I started off a little bit slow, especially returning - I was making a lot of mistakes," Keys told the WTA website. "But I felt like once I really got going and got a good feeling on the court, I played really well.
"My serve got me out of some tough positions in the first set and really helped in the second set.
"I've really enjoyed being in Japan. I don't mind the heat and I like the surface - it's really fast - so it works for my game."
Zheng Jie won the last eight games as she came from a set down to beat ninth seed Kristina Mladenovic 6-7(5) 6-2 6-0.
"The first set was really close - I think in the most important points I didn't play so well," Zheng said. "She was serving so big. I had never played against her in singles before and she was so tough.
"Early in the second set I started to play more aggressively against her, and I broke her early in both sets, and that helped me because she started thinking about it more. It was very good to break early."
Zheng will face Keys - 12 years her junior - in the quarters.
Japan's Misaki Doi beat 43-year-old compatriot Kimiko Date-Krumm 7-6(3) 6-4 to make it into the quarter-finals.
There was nothing to separate the two players in the opening set, with both enjoying several chances to clinch it.
Doi improved her game in the second set, however, not giving up a single break point while being ruthless about taking advantage of the Japanese number one's weak second serve to earn the break that gave her victory.
Next up for Doi is Australian Sam Stosur.
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