Sharapova broke the world number one in the first game and mounted a stiff challenge in the opening set but wilted in the second to extend a losing record that goes back 12 years.
Williams said the Russian brings out the best in her.
"When I play her, I know automatically I have to step up my game," Williams said. "I think that makes me play better. When I'm forced to play better, I do well."
The gulf between Williams and Sharapova, a former number one and five-times grand slam champion, speaks volumes about the balance of power in the women's game.
Williams has reached the last four at Melbourne Park on six previous occasions and gone on to win the title every time.
On Tuesday's evidence, it looks like only illness or injury can prevent the 34-year-old from repeating the feat and adding a 22nd major title to her collection.
The American, who came into the tournament after a four-month layoff, twice had treatment for "food poisoning issues" during breaks in the 92-minute rematch of last year's final.
Sharapova handed back her early break with two double faults in game four but started the match well and offered a genuine test in the 55-minute opening stanza.
The Russian showed character to storm back from 0-40 down to hold serve for 4-4 and was all over Williams's serve in the next game, the American yelling "C'mon!" on successive points as she finally held.
The match turned on the next game when Sharapova fended off three set points despite failing to get her first serve in before Williams converted the fourth after a long rally with a volley at the net.
Sharapova had fired 21 aces in her fourth round match but managed just seven against Williams.
"I think if you're serving maybe 180 (kph) against somebody else compared to Serena, that's an ace," said the 28-year-old.
"Against Serena, as we all know, the return is one of her great strengths. She's very explosive. She stays quite close to the baseline.
"She cuts the ball early. She doesn't give you many angles."
Williams has an 8-0 record against her semi-final opponent Agnieszka Radwanska, who as the fourth seed is the highest ranked challenger remaining in the draw.
"Nothing's guaranteed in sports," said Williams. "I still have to win two matches against potentially two extremely tough opponents."
(Editing by John O'Brien/Peter Rutherford)
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