Jankovic has suffered leg and ankle injuries in recent weeks
Top seeds Jelena Jankovic and Caroline Wozniacki were both beaten in the third round of the Cincinnati Open.
Qualifier Akgul Amanmuradova of Uzbekistan stunned Jankovic 7-6 (7-2) 6-4 after 16th seed Marion Bartoli had earlier beaten Wozniacki 6-4 6-1.
Kim Clijsters was the only one of the top nine seeds to reach the last eight, beating unseeded American Christina McHale 6-1 6-1.
Ana Ivanovic and Maria Sharapova were among the other winners.
The 114th-ranked Amanmuradova hit 12 aces as she sent defending champion Jankovic crashing out.
"Last year, I lost in the first round," said Amanmuradova. "I was just hoping to pass through the qualifier and see what happens. Today was one of my best games ever. I'd never beaten a top-10 player.
"Serving a lot of aces helped me save my energy."
I think the depth of the game is a lot deeper than it was a few years ago
Jankovic admitted: "The key to today's match was I really didn't return well and her serves were really accurate. I didn't really have many chances."
Ivanovic kept up her good run with a 6-0 6-3 win over Russia's Elena Vesnina as she looks to make her way back from 62 in the world rankings.
And the Serb credits her current good form to her new physical fitness coach Marija Lojanica, who works with the Serbian Fed Cup team.
"I'm much better physically at the moment and I feel stronger on the court," said the former world number one.
"So that puts me in better position to hit the ball, which then I can generate more power and hurt opponents more. So it's kind of a circle. It's all kind of connected.
"That's what I've been always saying in past few months. It was always this balance, my tennis, fitness, it was always, you know, playing catch up in some ways.
"But now I feel it's sort of all marrying together, and it's starting to work and click really well."
Tenth seed Sharapova saw off seventh seed Agnieszka Radwanska 6-2 6-3, and later said she was not surprised by the number of unexpected results.
"I think it just speaks a lot about the depth of the game right now," said the Russian.
"I think the depth of the game is a lot deeper than it was a few years ago, where maybe in the first few rounds you could have gotten a few easy matches, easy scorelines.
"I think now, from the get-go, you really have to be ready for the first round."