Remarkably, for a player who struggledto get her first serve over the net for much of the match, Williams suddenly unleashed a 125mph rocket - the second fastest recorded serve in the women's game.
She holds the record at 127mph, achieved in Zurich in 1998, and this latest howitzer gives her top spot on the official hit-list which is now restricted to the stadium courts at the major tournaments where the speed guns are used.
Williams said: "I didn't even know I had served one at 125mph because I was just getting ready for the next serve when I saw it on the display.
"I used to really try to serve hard but I am not doing that anymore and now it just comes out fast."
Williams now plays fifth seed Justine Henin-Hardenne, who took just 87 minutes to record a 6-2, 6-2 defeat of Virginia Ruano Pascual.
She had every right to be amazed at her service speed because the rest of the match featured a variety of serves that sometimes hit halfway up the net, clipped the tape before falling over the other side, or landed in court without real power.
At times, her first serve success was less than 30 per cent - when they did land in she won a staggering 87 per cent of the points.
It is an aspect of her play that keeps the critics wondering just how good the elder Williams sister really is following last year which saw her beaten in three successive Grand Slam finals by Serena.
It is two years since Venus won a Grand Slam - the US Open - and unless she starts to find consistency in her service game and forehand groundstrokes then the Australian title will go to Serena. Neither sister has triumphed in Melbourne and that is why they have both turned up looking for glory.
Serena opted to take part in the Hopman Cup in Perth to ensure she was up to speed while her sister arrived Down Under the Wednesday-before the tournament started. Venus today admitted that her sister had been "too good and everyone tried to take her down last year and no one could do it".
Venus added: "If I have to play Serena in the final then I will be trying to survive out there but we have only reached the semi-finals.
"I don't think about who I could meet in the final because if I don't make it then what's the point? I am just trying to get there myself and then I will look at who is across the net.
"I have to focus on keeping improving. What I was happiest about today was that most of my errors were from hitting the ball long rather than into the net. I don't like that."
It was always going to be a question of which Williams sister wanted a first Australian title most. On form it is Serena, although she has also been far from unbeatable with errors regularly mixed in with thunderous groundstrokes.
Another all-Williams Grand Slam final - the fourth in a row - would give credence to the view that the game is now all about two American sisters. The next couple of days will be illuminating.
