CHARLESTON, SC, USA -- 2011 Volvo Car Open champion Caroline Wozniacki of Denmark moved into the quarterfinals of the Charleston tournament for the sixth time in her career with a grueling 6-4, 3-6, 6-3 victory over No.12 seed Mihaela Buzarnescu of Romania on Thursday.

"I started off very well," Wozniacki told the press, after her win. "I started off aggressively. I played the way I wanted to play. I could feel the pressure coming from her, so I knew she wasn't going to give up easy, and she played even more aggressively, and things started turning her way a little bit."

"In the third set again I got a good start but wasn't really feeling it," Wozniacki continued. "Then at 3-3 it was just, I have to step in, I have to try and take the ball earlier, and I think that paid off a little bit."

No.5 seed Wozniacki had won both of her previous meetings with Buzarnescu in straight sets, in the first round of both the 2017 US Open and the 2018 Australian Open -- the latter of which Wozniacki went on to win for her first Grand Slam title. Buzarnescu pushed the Dane to three sets for the first time, but Wozniacki was still able to prevail after two hours and 35 minutes of grinding play.

World No.30 Buzarnescu was a risk-taker on the day, with 39 winners and 47 unforced errors. Wozniacki was steadier and had a cleaner ratio, with 26 winners to 22 unforced errors. Wozniacki was also stronger when she had break points, converting seven of 12, while Buzarnescu could only win six out of her 17 chances.

In the elite eight, former World No.1 Wozniacki will now take on No.15 seed Maria Sakkari of Greece, who pulled off a stunning upset of No.2 seed and defending champion Kiki Bertens earlier on Thursday.

"I'm very familiar with [Sakkari]," said Wozniacki. "We've practiced together in Monaco quite a bit, so I'm familiar with her and her game. She obviously loves the clay and it's going to be a little bit of a different match because she's a right-hander and she puts a lot of spin on the ball, but at the same time it's going to be similar in the way [Sakkari and Buzarnescu] like to play on clay and use the angles."

Wozniacki kicked off the match strongly, holding at love to start the tilt, then hitting a divine dropshot winner on break point in the next game. The Dane earned a second love hold to reach 3-0, closing out that game with an ace.

Buzarnescu put her game into gear mid-set, outlasting Wozniacki in a nearly 13-minute game to hold for 4-2, saving three break points in the process. The Romanian powered through rallies with her backhand to stay within touching distance, and was rewarded with a service break to get back on serve at 5-4, finishing that game with a forehand winner.

Nevertheless, the momentum was short-lived, as Buzarnescu hit errors to squander a 30-0 lead in the following game, leading to a set point for Wozniacki. A long forehand miscue by Buzarnescu ended the opening frame, and Wozniacki had a one-set advantage in her pocket.

But Buzarnescu rebounded in the second set. After an early exchange of breaks, the Romanian picked up a crucial service break at 2-2, punching a backhand swing volley for a winner to attain the advantage. Buzarnescu blasted a forehand winner on the sideline to fend off a break point in the next game, then consolidated for 4-2 with an ace on game point.

The Romanian’s down-the-line shots served her well in the second set as she never ceded her lead through 5-3. In that game, Buzarnescu quickly got to triple set point on Wozniacki’s serve; the Dane saved two set points with aggressive play, but she shot a backhand error long on the third, giving Buzarnescu her first set in their professional head-to-head.

Wozniacki shook off the second set successfully at the start of the decider. The Dane broke Buzarnescu at love in the opening game, staved off three break points to hold for 2-0, and then garnered a second service break for 3-0 after Buzarnescu hit consecutive double faults to drop serve.

Buzarnescu, however, was not done yet. She broke Wozniacki for 3-1, and then powered through a 9-minute game with incredible backhands to eke out a hold. On a roll, the left-handed Romanian clinched another break for 3-3, as a frustrated Wozniacki let miscues fly wide en route to a dead-even final set.

But the former World No.1 amped up her game when she needed to most, crushing crosscourt forehand winners to break for 4-3, then holding at love for 5-3. Finally, the Buzarnescu magic wore off, as Wozniacki dominated with her backhand to reach triple match point. The first chance was converted for the Dane when a Buzarnescu backhand went long.