The American raced to the finish line 6-3, 6-2 in just 66 minutes on Margaret Court Arena as temperatures began climbing towards a forecast 40C.
She will face defending champion Madison Keys for a place in the quarter-finals.
"Nice to be first out on court and not a super long match and didn't need to kill myself in the heat today," she said.
"I felt like I played pretty well. I like to stick to a strategy ... lucky that everything worked out well."
Selekhmeteva bundled out last year's semi-finalist Paula Badosa in the previous round but was no match for Pegula, who made the last eight at Melbourne Park in 2021, 2022 and 2023.
Pegula grabbed a break in game five of the first set with a forehand winner to take charge and never relented, with the Russian making 16 unforced errors.
The decisive break in the second set came in game six, with another slew of errors from Selekhmeteva leading to her downfall.
Pegula is into a Grand Slam last 16 for the 11th time but she has only pushed on to a final once before, in 2024 at the US Open when she lost to Aryna Sabalenka.
