Williams attempted a brave comeback in the third set, but it wasn't quite enough.
For Williams it only the second time in her 24-year professional career, which encompasses more than 1,000 matches, that she has lost to a player outside the top 150.
Defeats against rank outsiders just don't happen; dating back to 1994, the legendary American had only fallen seven times to players that sit outside the top 100 in the world, though the level of Andreescu was way above that.
After her stunning win over Wozniacki, backing it up was meant to be a difficult task for Andreescu.
She'd had back problems on Thursday night, and had already played five matches in Auckland.
And the 18-year-old qualifier was in rarefied air; in her professional career spanning 134 matches, she had only had two wins over a top 50 player, and one was against Wozniacki.
Williams made a bright start, while Andreescu initially struggled to reach the levels of Thursday night.
The American used all of her experience to good effect, pinning the Canadian beyond the baseline and inducing errors.
But her dominance was short lived. Both players traded breaks in a tight first set, and a tiebreak seemed inevitable.
Williams was completely dominant in the tiebreak reeling off seven consecutive points after dropping the first.
It looked ominous for the Canadian, especially when she was broken in the first game of the second set.
But she rebounded spectacularly, finding herself back 'in the zone', as she recaptured the magic of Thursday night.
She broke Williams on three consecutive occasions, wrapping the set up 6-1 before the crowd seemed to realise what was happening.
The pattern continued in the third set, with Williams unable to do anything to stem the bleeding, though her late revivial was typically brave.
