Sakkari stuns defending champion Bertens at Charleston

World number 50 Sakkari showcased supreme court coverage skills throughout the almost two-hour match, which she clinched after breaking the second-seeded Dutchwoman, who was playing on her preferred clay surface, for the seventh time in the contest.

Sakkari, the 15th seed, saved four set points in the opener and held steady the rest of the way to secure her third career top-10 victory following wins over Caroline Wozniacki in 2017 and Karolina Pliskova last year.

The win also righted the ship for Sakkari, who after a strong start to the year that included a run to the third round at the Australian Open, fell in the early stages of tournaments is St. Petersburg, Acapulco, Indian Wells and Miami.

"I'm pretty excited and speechless," Sakkari said on court. "I had a rough two months after the Australian Open, I was losing in the first round, second round and I really wanted to come back stronger.

"Finally I came back and I'm feeling great."

Up next for Sakkari will either be fifth seed Wozniacki or 12th-seeded Romanian Mihaela Buzarnescu.

Earlier, Swiss ninth seed Belinda Bencic extended her fine form with a 6-2 7-5 win over Taylor Townsend to reach the quarter-finals while Croatia's Petra Martic beat Jessica Pegula 2-6 6-3 6-2.

World number 21 Bencic, who entered the match having lost only six games all week, grabbed eight of the opening nine points to set the tone early in what proved to be a routine victory.

"The key was to put pressure on her. She's got spin that can make you uncomfortable. If you give her time, she's going to spin you off court and make you run," said Bencic. "I just tried to play with smart tactics."

Top seed Sloane Stephens will kick off the evening session against Australia's Ajla Tomljanovic.