Venus Williams moved within two wins of a possible sisterly showdown with Serena in the semi-finals of the U.S. Open by beating German 26th seed Laura Siegemund to reach the fourth round on Saturday.

The sixth-seeded Williams came up big on the critical points to make quick work of Siegemund in a 6-1 6-2 victory in Arthur Ashe Stadium.

The 36-year-old Williams, oldest player in the women's draw, repelled seven of nine break points held by the German, and capitalised on six of 10 of her own break opportunities.

"Just doing what I have to do, being able to dominate when I need to," Williams told reporters.

"Being in control of the points, so even if I lose a point, I feel like, OK, I'm in a position to win the point. That's where I want to be every single time."

Williams, a seven-times grand slam singles winner and twice U.S. Open champion, has enjoyed some recent success, reaching the semi-finals at Wimbledon and the final at Stanford.

Siegemund appeared poised to offer the American a good test, coming off a productive stretch in which she made the semi-finals in Bucharest, won in Bastad and reached the quarter-finals at the Rio Olympics.

But she wilted under the raw power of Williams, who forced her into 32 errors and was gifted with 26 unforced miscues in the 78-minute match.

That moved Williams into a round of 16 clash against big-serving, 10th-seeded Czech Karolina Pliskova, a 6-2 6-4 winner against Russian 17th seed Anastasia Pavlyuchenkova.

Top-seeded Serena advanced to the fourth round with a 6-2 6-1 demolition of Sweden's Johanna Larsson.

Pliskova showed her prowess last month by winning the title in Cincinnati, where she beat Angelique Kerber in the final to deprive the German of overtaking Serena as world number one.

"Every match is a little different," Venus said. "Today, my opponent tried to play aggressively, but I played that game just a little bit better than her."

As for Pliskova, the elder Williams said: "We play kind of a similar game. So it's about one of us playing that game better."