Serbia's former world number one, who had a bye in the first round, looked out of sorts from the start, committing numerous errors and winning only 43 percent of her first serves. She was beaten in less than an hour by the 268th-ranked Diyas.
The Kazakh qualifier, who turned 17 three days ago, was as surprised as everyone by her lopsided victory. "She is a top player and I never played someone like her before," Diyas said. "But as the match progressed I started feeling more and more confident and my nerves were gone by the first set."
Diyas, who had not won a tour match this year before this week, now takes on sixth-seeded Maria Kirilenko in Friday's quarterfinals after the Russian whitewashed Ukraine's Kateryna Bondarenko 6-0, 6-0 in the earlier match on centre court.
Earlier, Victoria Azarenka of Belarus beat Germany's Andrea Petkovic 6-4, 2-6, 6-1 and eighth-seeded Spaniard Maria Jose Martinez Sanchez saw off Ukraine's Alona Bondarenko 6-4, 1-6, 6-1 to reach the last eight.
Meanwhile, the tournament suffered a double blow on Wednesday when top seed Nikolay Davydenko was beaten in his opening match while champion Mikhail Youzhny was forced to pull out with a virus infection.
“I had made a decision to withdraw after losing my doubles match,” Youzhny, seeded second, told reporters.
“I don't feel well at all and I don't think I would have been able to play the singles in such condition. I will now have to take a break from training for a few days and see how I feel,” added the world number nine, still with a chance to qualify for the World Tour Finals in London.
Davydenko, who like his Russian compatriot Youzhny had a bye in the first round, lost his opening match for the second week in a row, going down to Uruguay's Pablo Cuevas 7-6, 7-6.
Jankovic’s loss and Wednesday's double setback has added to the disappointments of the home crowd and local organisers of the annual ATP and WTA tournament that had been decimated by the withdrawals of high-profile players, particularly on the women's side.
Russians Vera Zvonareva, Svetlana Kuznetsova and Maria Sharapova and 2009 champion Francesca Schiavone all pulled out. Serbia's eighth seed Janko Tipsarevic, who lost to Youzhny in last year's final, was bundled out in the first round on Wednesday by Horacio Zeballos 4-6, 6-4, 6-3.
Top seeds Rohan Bopanna of India and Aisam-Ul-Haq Qureshi of Pakistan entered the quarters. The Indo-Pak pair edged out Czech Republic’s Leos Friedl and Victor Hanescu of Romania 6-1, 3-6, 10-8.