Win, and Williams will advance to the semifinals, where Karolina Pliskova awaits.

MORE: Must-see Open images | Andy Murray bids for men’s semis

Wednesday was the final day for quarterfinal competition in both women’s and men’s play at the Open. Williams faces Halep, the No. 5 seed, at 7 p.m. ET. All remaining matches in singles competition are at Arthur Ashe Stadium, the main show court of the USTA Billie Jean King National Tennis Center in Queens, N.Y.

Pliskova, a Czech and the tournament’s 10th seed, dispatched 18-year-old Ana Konjuh of Croatia 6-2, 6-2 in Wednesday’s first match.

Williams entered her match against a formidable and familiar foe. In nine matches, she won seven against Halep. She also remains the biggest force in women’s tennis, while Halep has yet to get a breakthrough victory.

But Halep is dangerous. She had a strong summer and is in top form.

“My game now I feel is the best that I had ever, because I feel strong on my body. I feel that I can control every tactic during the match,” Halep said after Monday’s fourth-round victory. “I can change. I’m able to the change the tactic when I need.

“It’s tough to talk about the winning a Grand Slam because I never won one, so I don’t know the feeling. But I’m working hard to reach that title, to make that dream come true. So I’m just working, dreaming about it, and if one day it happens I will be very happy.”

Where Halep has no Grand Slam titles, Williams has 22 and a record number of singles win in major tournaments.

“She’s the best player in the world,” Halep said. “I don’t have to be afraid or to have emotions because I have nothing to lose.”

So there is always danger for Williams.

“With everyone I play, they play a step up and above their ranking. I think that’s the beauty, one of the reasons I’m able to hang. Everyone I’m playing is playing like they’re No. 1,” Williams said. “To me, it doesn’t really matter who I play, because I have to expect they’re going to play the match of their life. That’s how I go into these matches now.”

The women’s semifinals will be played Thursday night. The championship match is scheduled for 4 p.m. ET Saturday.

The second semifinal pits No. 2 Angelique Kerber against unseeded Caroline Wozniacki.