World number one Serena Williams fended off spirited Simona Halep on Wednesday, downing the fifth-seeded Romanian 6-2, 4-6, 6-3 to reach the US Open semi-finals.

Williams, seeking a record seventh US Open title and 23rd Grand Slam crown, fired 18 aces en route to the triumph, which set up a Thursday evening meeting with first-time Grand Slam semi-finalist Karolina Pliskova for a place in Saturday's championship match.

But she made her task against 2014 French Open finalist Halep harder with 43 unforced errors and Halep, firing on all cylinders from the baseline took advantage.

The Romanian became the first player in the tournament to break Williams's serve, and the first to take a set off the top seed.

After failing to convert any of a dozen break points in the 65-minute second set, Williams regained control in the third with a break for 3-1 and steamed home from there.

"I'm glad I got tested," said Williams, who saw seven break point opportunities go by in the second game of the second set.

She had five more chances in the final game of the set, which went to deuce eight times before Halep closed it out on her fifth set point when Williams fired a backhand long.

"I think her level really picked up in the second, but I had some opportunities that I didn't take.

"So what I gather from that is I really could have played better in that second set and maybe had an opportunity to win in straights.

"If anything, that's the biggest silver lining I take."

She can also take heart from her ability to produce some massive serves when she needed them, especially after a sore right shoulder prevented her from even practicing her serve much in the build up to the tournament.

"I feel OK," Williams said. "I'm not at 100 percent, but I'm OK. Nothing to complain too much about.

Halep was left regretting her inability to convert two break points in the opening game of the third set.

"(If) I could take that game, I think the story of the match would have been different," Halep said. "I am a little bit disappointed that I had could not take the chances, but I'm also happy about the way I stayed there and played."

Williams's semi-final against Pliskova promises more fireworks. The 10th-seeded Czech leads the WTA tour in aces this year.

After surviving a match point en route to a fourth-round victory over Venus Williams, Pliskova beat 18-year-old Croatian Ana Konjuh 6-2, 6-2.

"I'm so excited to be in my first semi-final," said the 24-year-old Pliskova, who had failed to make it out of the third round in 17 prior Grand Slam appearances.

Having broken through to the quarters Pliskova didn't waste her opportunity.

With a quick break under her belt in the opening game she was able to swing freely and needed just 57 minutes to subdue Konjuh, who upset fourth-seeded Agnieszka Radwanska in the fourth round.

"I think it was the serve," said Pliskova, who finished off the match with two of her three aces. "My serve was very good today."

Konjuh, the 18-year-old who is projected to rise from 92nd in the world to 51, couldn't be disappointed with her fortnight's work.

"I cannot be sad after all of this," she said. "When I came here I could only imagine playing the quarters. I think it's been a great tournament." 

- No. 1 ranking under threat -                     

Despite her dearth of Grand Slam success, Pliskova is playing with confidence after winning her biggest title to date on the hard courts of Cincinnati.

She beat Angelique Kerber in the final to deny the German world number two's bid to supplant Serena atop the world rankings.

Serena said she hadn't talked to her sister yet about how best to approach Pliskova.

"I haven't even gotten that far yet, actually," she said. "I'm still trying to just get over this match. I'm sure Venus will tell me something, but I'll be there. I'll be ready tomorrow."

Kerber, who booked her semi-final slot on Tuesday with a 7-5, 6-0 victory over 2015 finalist Roberta Vinci, can still nudge Serena from the summit.

No matter the outcome of Kerber's semi-final clash with former world number one Caroline Wozniacki, Serena must now reach the final to stretch her reign to a record-breaking 187 weeks.

If Kerber advances to the title match, Serena will have to beat her for a seventh US Open trophy to stay number one.