The 31-year-old American was later joined in the final four by Belarusian world number two Victoria Azarenka, who booked her spot with a testing 6-3 7-6 (5) win over 10th seeded Dane Caroline Wozniacki.
Williams's devastating form suggests that there is little standing in the way of the muscular American from adding the prestigious Cincinnati hardcourt event to her long resume.
In a career that has generated 54 singles titles, including 16 grand slams, Williams has won just about everything there is to win in her sport but Cincinnati, one of the WTA tour's most prestigious tournaments, is not among them.
With Friday's rout, Williams sets up a semi-final date with defending champion Li Na of China, who received a walkover into the last four when Poland's Agnieszka Radwanska withdrew from the tournament to attend her grandfather's funeral.
Williams was in ruthless form to open the match, steamrolling past her opponent to take the first set in just 16 minutes.
Halep offered token resistance in the second but there was never any doubt Williams would prevail as she went on to wrap up her 59th match win of the season in just over an hour.
With eight titles and 10 finals appearances from 11 events this season while losing only three matches, it would be hard to defect any flaw in Williams's game.
Certainly Halep, who beat Marion Bartoli in what was the final match of the Frenchwoman's career before her shock retirement announcement on Wednesday, never threatened to send Williams to a fourth defeat.
Though the American's performance in Cincinnati has impressed nearly everyone else, under her own critical eye it has been something of a disappointment.
"I definitely haven't felt good all week, so surprised I'm still in the event, to be honest," Williams told reporters. "It's not really about how you play at the end ... it's about being able to win, whether you play good or bad.
"I was able to so far and be in the semi-finals. So, you know, considering, it's a good result. I obviously want to do better, and I'll just see how it goes."
Earlier, 14th-seeded Serb Jelena Jankovic, the 2009 Cincinnati champion and runner-up in 2011, needed just 63 minutes to book her place in the last four as she pounded 12th seed Roberta Vinci of Italy 6-0 6-4.
"I started aggressive and I felt the ball great," said Jankovic, who will take on Azarenka for a place in the final. "I'm really pleased with my performance today, so that's important.
"I love playing here in Cincinnati because I have great results in the past, I was a finalist and a champion before so you always love coming back to places where you can win."
(Editing by Mark Lamport-Stokes / Ian Ransom)
