NEW YORK, NY, USA -- Six-time US Open champion Serena Williams booked a spot in the US Open quarterfinals after a 6-3, 6-4 Sunday afternoon victory over No.22 seed Petra Martic of Croatia on Arthur Ashe Stadium.

"I feel fine right now physically, and obviously my state is super intense," Williams said during her post-match press conference. "I'm always incredibly intense. Just trying to keep moving forward."

In the first meeting between the two Top 25 players, No.8 seed Williams stayed steely, overcoming the solid play of Martic and a late ankle turn which required a medical timeout to notch the victory after an hour and a half of play.

"Ankle, I usually know if it's horrible early on," Williams stated. "I had a really bad ankle sprain in January. I was like, instantly, 'No, this can't happen. I'm finally healthy.' But I'll see tomorrow. So far I'm good. I have been managing it. We'll see tomorrow."

Williams improved her stellar record in US Open fourth-round matches to 16-2, and now has made it to the quarterfinals in her last 11 appearances at her home Grand Slam event. The American legend won nearly 80 percent of points off of her first serve, and was clinical with her shots, blasting 38 winners to just 19 unforced errors.

Martic is having a career-best year, winning her first WTA singles title at Istanbul and reaching her first Grand Slam quarterfinal at Roland Garros. However, the 28-year-old was only able to muster up 11 winners on the day, and only converted one of her three break points, in the very first game of the clash.

23-time Grand Slam champion Williams will now face No.18 seed Wang Qiang of China in the elite eight. Wang moved into her first-ever Grand Slam quarterfinal after upsetting No.2 seed and reigning Roland Garros champion Ashleigh Barty in straight sets earlier on Sunday.

"Every single match I have played, people come and they plan their best," said Williams. "For me, I have to be the greatest whether it's against the second seed, the No. 1 seed, or the No. 80th player in the world. I have to show up or else I'm going to go home."

"I feel like in this tournament, I guess, I have definitely turned a different zone," Williams added. "I'm not sure if I can articulate what zone that is. But it's not about kind, it's not to myself, it's not about anything in particular. It's definitely something different, though."

Martic made her mark on the match immediately, winning five straight points and breaking Williams via a deep forehand return after the American had a 40-0 lead in the first game. Williams, however, swiftly struck back, erasing three game points Martic held in the next game before breaking for 1-1 with a forehand winner down the line.

The Croat fended off another break point at 2-2 but was unable to get a look in Williams’s next two service games, as the American edged to a 4-3 lead. There, Williams again pulled herself into contention in another game where Martic had game points, blasting a return winner to set up break point. Williams converted that chance with a winning rally backhand to lead 5-3.

Serving for the set, Williams fired errors and a double fault, giving Martic two chances to put the set right back on serve. Powerful groundstrokes from the long-time former World No.1 whisked away those break points, though, and Williams reached set point after Martic missed a service return at deuce. A deft dropshot on Williams’s first opportunity clinched her the one-set lead.

At 2-2 in the second set, Williams rolled her ankle while misfiring on a volley, causing concern, but the American still found a blistering backhand and a drop volley winner in succession to earn a crucial break and lead 3-2.

The American had a medical timeout to have the trainer review her ankle after that game, but appeared unfazed as she found strong serves to wrap up a close game and hold for 4-2. Martic then found herself in dire straits on serve in the next game, but escaped four break points to keep herself just a single break behind at 4-3.

Martic, though, was unable to grasp another break point, and Williams methodically pulled to double match point at 5-4. The 23-time Grand Slam champion slammed her fourth ace of the match to end the affair and clinch an astonishing 52nd victory in fourth-round matches over all of the Grand Slam events during her career.