Williams overcomes gutsy Halep to make semi-final

Williams seemed on course to be in for an early night after breaking Halep twice to win the opening set 6-2, but Halep dug deep in the second set and hung on to a single break of serve to take it to a decider.

There was no denying Williams, however, the crucial break coming at 2-1 in the third set and the world number one held out for the win.

She now faces surprise semi-finalist Karolina Pliskova in the semi-final on Friday.

"There will be a lot of aces in that match I think," said Williams, who laid down 18 aces in the quarter-final win over Halep.

"I haven't played against many big servers recently but she places her serve really well. She definitely gets some speed on it but her placement is really, really nice."

There is no calendar slam up for grabs this year but history continues to beckon Williams, who would stand alone as the greatest grand slam champion in the Open Era if she can claim a 23rd title.

That would move her past Steffi Graf and leave her one shy of Margaret Court at the top of the all-time list.

While Williams will compete in her 33rd grand slam semi-final, the tie is further uncharted territory for Pliskova, who arrived as the only top-20 player to have not reached the fourth round at one of the four blue-riband events.

The other semi-final is an equally attractive match-up pitting unseeded former world number one Caroline Wozniacki against second seed Angelique Kerber, who could leave New York with her second grand slam title and snatch Williams' number one ranking.

Australian Open champion Kerber holds a 7-5 lead in head-to-head meetings against the Dane, with their last two hardcourt encounters split.

The 26-year-old Wozniacki, meanwhile, is still chasing an elusive maiden grand slam title but has come close at Flushing Meadows, reaching the semi-finals on five occasions and the finals twice in 2009 and 2014.