Diyas served for the match at 6-5 in the second set but was broken and lost the tie-break. However, she collected herself and from 2-2 in the third set won four of the next five games to close out the 6-2, 6-7(3) 6-3 triumph.
"She was a Wimbledon semifinalist and is a great player, so this helps my confidence," Diyas said. "I was trying to move her, I tried to go to the net and I served pretty well."
"It was a very tough match and it's a big win," Diyas said. "I'm really happy. I played well."
Monday's other surprise saw Casey Dellacqua overcome a set point and a 5-2 deficit in the first set of her 7-5, 3-6, 6-3 victory over fifth-seeded Elena Vesnina. She closed out the match in style, winning the last eight points from 4-3 in the third set.
One seed did advance, with No.8 seed Alison Riske a 6-3, 6-3 winner over Teliana Pereira. Up next for her is fellow American Madison Keys, who was a 7-6(2), 6-2 victor over María-Teresa Torró-Flor. Riske and Keys just faced off last week in Rome, with Keys triumphant in straight sets.