Watson followed up her victory over fifth seed Sloane Stephens with a 7-6 (7-0), 6-2 win over Italian ninth seed Roberta Vinci and has yet to drop a set in the tournament.
The world No 49 served three aces and won 76 per cent of points on her serve in a hard-fought opening set before racing through the tie-break without dropping a point.
And the statistics were even more impressive in the second, Watson sending down five aces and claiming 84 per cent of points behind her first serve as she broke Vinci three times on her way to victory in an hour and 47 minutes.
Watson will face American Alison Riske in the last four after the eighth seed beat second seed Zarina Diyas 6-3, 6-0.
The tournament saw its top three seeds crash out yesterday, with top seed Casey Dellacqua beaten in the second round before Diyas and third seed Camila Giorgi bowed out in the quarter-finals.
Japan’s Kurumi Nara defeated Giorgi 6-1, 3-6, 6-2 to book a semi-final with qualifier Madison Brengle, who saw off Dellacqua’s conqueror Karin Knapp 6-7 (3-7), 6-4, 6-4.
Meanwhile, Juan Martin del Potro’s comeback run at the Apia International ended in a quarter-final loss to Mikhail Kukushkin in Sydney.
Del Potro exceeded his own expectations after more than ten months out with wrist problems by beating Sergiy Stakhovsky and top seed Fabio Fognini to reach the last eight.
But qualifier Kukushkin came out on top in two tie-breaks, beating the defending champion 7-6 (7-5), 7-6 (7-3).
Del Potro was nevertheless content with his week’s work ahead of the Australian Open, saying: “It was a tough match. I think he deserved to win because he played better than me in the tie-break. I served well, but my wrist hurts a little bit more than yesterday, and I couldn’t hit harder than my last match.
“I think anyway it’s a great week for me. I played three matches in a row and I played against good players, which means a good comeback for me.”
In the last four, Kukushkin will play fifth seed Leonardo Mayer, who saw off fourth seed Julien Benneteau 6-3, 7-6 (7-4).
The other semi-final will be between Luxembourg’s Gilles Muller and Serbian qualifier Viktor Troicki.
Muller ended the run of home hope Bernard Tomic, the champion in 2013 and runner-up last year, with a 7-6 (7-3), 7-6 (15-13) victory, while Troicki fought back from a break down in the deciding set to beat Simone Bolelli 6-3, 3-6, 7-5.
In the women’s tournament, big-hitting Karolina Pliskova continued to blast her way through the draw.
The Czech reached the first WTA Premier final of her career with a 6-3, 6-2 victory over fifth seed Angelique Kerber.
Pliskova’s opponent in the final will be second seed Petra Kvitova, who avenged defeat at the same stage last year by Tsvetana Pironkova.
The Bulgarian went on to win the title 12 months ago but could not pull off a repeat as Kvitova emerged a 7-5, 6-1 winner.
Frenchman Lucas Pouille eased into the semi-finals at the Heineken Open in Auckland with a 6-3, 6-2 victory over Albert Ramos-Vinolas to set up a clash with countryman Adrian Mannarino, who defeated Alejandro Falla 6-3, 6-1.
Czech Jiri Vesely is also through to the semis after beating Donald Young 6-2, 6-3.
He next meets fourth seed Kevin Anderson, who defeated American Steve Johnson 6-4, 7-6 (10-8).