Williams, the 17th seed, saw off 27th seed Pavlyuchenkova 6-4 7-6 (7-3) in the first of the quarter-finals.
The 36-year-old American has never won the title, her best effort a runner-up finish to sister Serena in 2003.
She goes on to face Spain's Garbine Muguruza or fellow American Coco Vandewehge in the last four.
"Whoever wins I'm just happy I have an opportunity to play again," said Williams, who is the oldest woman to reach a Grand Slam semi-final since Martina Navratilova at Wimbledon in 1994.
"That's all I'm focused on."
She added: "I'm so excited, today was such a hard-fought match. She never let up.
"It's wonderful to be here at the start of the year, I want to go further. I'm not happy with this."
On Wednesday, Serena Williams will play Britain's Johanna Konta at around 02:00 GMT, following the match between Czech fifth seed Karolina Plisokva and Croatia's Mirjana Lucic-Baroni.
Venus Williams reached the Australian Open semi-finals for the first time in 14 years with a straight-set win over Russia's Anastasia Pavlyuchenkova.