The 36-year-old overcame the Russian 6-4, 7-6 (7/3) and will play either fellow American Coco Vandeweghe or French Open champion Garbine Muguruza for a place in the final.
Her achievement makes her the oldest woman to reach a Grand Slam semi since a 37-year-old Martina Navratilova at Wimbledon in 1994.
It is a remarkable feat for Williams, who made her professional debut in 1994 and is enjoying a late-career renaissance following a battle with a rare autoimmune disorder.
Henin is long retired but Serena is still going strong and plays her quarter-final, against Johanna Konta, on Wednesday, with an all-Williams title match still on the cards.
More From This Section
Venus Williams also made the last four at Melbourne in 2001, losing to then world number one Martina Hingis, another veteran who is playing in doubles at this year's Australian Open.
"I'm so excited, she never let up. We have always had these quality matches. I'm not happy just with this but so happy I can go further," she said.
The seven-time Grand Slam champion is yet to drop a set. She has benefited from a favourable draw, playing two qualifiers and China's fifth ranked player Duan Yingying.
But Pavlyuchenkova, seeded 24, was a different matter, having beaten Elina Svitolina, seeded 11, and Svetlana Kuznetsova, the eighth seed, on her way to the quarter-finals.
- In the zone -
===============
Two netted forehands gave her a break point on Williams' opening serve as the match got underway, but she couldn't convert and it went with serve.
But 13th seed Williams gave her another chance in game three and this time she grabbed it with an untouchable crosscourt backhand.
It was short-lived as Williams struggled to find the mark on first serve and Pavlyuchenkova jumped on the second, with a netted forehand from the veteran again giving a break away.
Neither player was serving well but Williams began to assert herself, pushing Pavlyuchenkova around the court to create opportunities. She broke back for 4-4 with a volley after a high lunging forehand from the Russian.
Williams was starting to get in the zone, hitting some amazing winners, including an unstoppable backhand down the line to secure the set by breaking again to love.
A wild backhand in the next game handed momentum to the Russian, who broke for a 2-1 lead, but after a seesawing Williams broke straight back.
Another break each and it was 6-6 and into a tiebreak where Williams used her big-match experience to prevail, with Pavlyuchenkova succumbing on a double fault.