However at the same stage in set two at 3 all, Williams committed three additional forehand errors to lose serve and fall behind 3-4.
The American once more improved her play at a critical moment, striking several groundstroke winners to win eight points in a row to lead set to 5-4.
Pavlychenkova then held serve to level the set at 5 all.
Then serving at 30-0, Williams struck a down the line backhand winner which she followed up with an ace out wide to hold serve at love to lead 6-5.
The Russian recovered from 0-30 to hold serve in the following game with an assist from Williams as she struck three forehand errors.
In the tiebreaker, Williams trailed 1-3 before Pavlychenkova netted a backhand then double faulted to level the score 3 all.
A Williams down the line forehand return winner put the American ahead 4-3.
Williams struck an inside out crosscourt forehand winner which was followed by a Pavlychenkova doublefault to close the match.
In the end, Williams’ powerful, aggressive style of tennis had held up to serve her well as she had double faulted only once for the day while striking 35 winners to 29 unforced errors.
“I’m so excited. Today was such a hard-fought match. She never let up,” Williams said.
“It’s wonderful to be here at the start of the year. I want to go further. I’m not happy with this.”
In the semifinals, Williams will meet her fellow American, the unseeded CoCo Vandeweghe who upset the defending Australian Open champion Angelique Kerber of Germany in the fourth round before upsetting Roland Garros champion Garbine Muguruza in the quarterfinals.
Williams defeated Vandeweghe 6-4, 6-3 last year in Rome on clay in the duo’s only prior meeting.
Williams could possibly meet her sister, the second ranked Serena Williams in the Melbourne final round.