SAN JOSE, CA, USA - No.5 seed Donna Vekic booked a place in the last eight of the Mubadala Silicon Valley Classic with a 6-4, 6-3 win over Victoria Azarenka.

The No.5 seed rallied from deficits in both sets to see off the former World No.1, a winner at the event's previous iteration at Standford in 2010, who led 4-1 in the opening set and 2-0 in the second set.

"She started playing ubelievable and I was a little bit slow and behind on almost every ball. I managed to get into the match, thank God, just in time, and I'm happy with how I played today," Vekic said on-court after the match.

"I just tried to stay in the rallies and treid to make it a little bit more physical, try to get in the match, get some rhythm.

"I started playing better, feeling better, tried to go a little bit more for my shots, hit a little deeper and that was the key in the end."

Vekic won five straight games to end the first set, as Azarenka grew more erratic on serve and off the ground: the Belarusian hit five of her seven double faults for the match in the opener, including on set point.

In the second set, Azarenka broke in the opening game, but Vekic won three straight games to flip the set on its head.

The 23-year-old saved a break point in a key seventh game to keep her lead, and ultimately won the last three games of the match to seal victory in the first-ever meeting between the two. 

Vekic totaled 14 winners to 16 unforced errors in the match, while Azarenka racked up 26 unforced to just 13 winners, along with seven double faults.

The runner-up at the Citi Open in Washington, D.C. last year, Vekic elected to play the Premier-level event in San Jose for the first time in her career instead this week, and advances to her sixth quarterfinal of the season.

"It has always been a little bit difficult for me, playing in California - the balls are flying a little bit more," Vekic said.

"I'm trying to get adjusted to the conditions here and I'm playing better and better, so hopefully, I can stay here the whole week."

Up next, she will face American qualifier Kristie Ahn, who became the lone unseeded player to reach the last eight with a 6-3, 6-3 win over No.3 seed Elise Mertens. 

"I'm happy to be here for the first time. The crowd has been amazing so far. They're really supporting me, and I love to see that," Vekic added.

"I don't know a lot about how [Ahn] plays. She's a great girl, she's very nice off the court, so I'm looking forward to that match."