Serena holds off Halep to reach US Open semi-finals

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

But she made her task against the 2014 French Open finalist 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.

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

"I knew I could play a lot better," Williams said of her frustration in a second set that lasted 65 minutes. "She kept going for her shots -- she stepped it up." 

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 Pliskova, who had failed to make it out of the third round in 17 prior Grand Slam appearances.