The world number one looked to be approaching something like her best form ahead of her US Open title defence later this month as she fired down 12 aces and broke her opponent’s serve three times.

“It’s been a long time since the last time I played her (Vinci), but I think this is the best she’s played against me,” Williams told the WTA website.

“Her slice, I think I handled it really well tonight. More than anything, though, I think her serve was what kept her going really strong in the match tonight.”

She added: “I definitely feel it’s another step in the right direction. I don’t feel like I’m at my best or anywhere near it, but I’m going in the right direction.”

Second seed Simona Halep recovered from a sluggish start to defeat Polish sixth seed Agnieszka Radwanska and book a semi-final showdown with Italy’s Sara Errani.

Romanian Halep could barely put a foot right as she lost the first set 6-0 but recovered to secure a 0-6 6-3 6-1 triumph in a match lasting one hour and 40 minutes.

Italian 15th seed Errani enjoyed a much easier passage to the last four as she defeated Ukrainian Lesia Tsurenko 6-4 6-4.

Williams will face Belinda Bencic in the last four after the 18-year-old Swiss stunned fifth seed Ana Ivanovic 6-4 6-2 in Friday's fourth quarter-final.

Bencic reeled off six straight games from 4-4 in the first set to assume total control against the world number six from Serbia.

“I enjoyed it. It was amazing. The crowd was electric,” Bencic said in her on-court interview, reported by www.wtatennis.com.

“I’m in the semi-finals – I cannot believe it – I’m really looking forward to (Saturday).”