Sabalenka was the heavy favourite to beat unseeded American underdog Madison Keys and claim a third straight Melbourne title, but the instability of her second serve returned in a three-set defeat.
At the French Open she played what she described as the “worst final” of her life, hitting 70 unforced errors in windy conditions.
Her performance against Rybakina – who was bidding for a second major title after her 2022 Wimbledon win – was far from disastrous.
In fact, there had been signs she was maintaining her composure until fifth seed Rybakina broke back in the deciding set.
Out of nowhere, errors streamed from both wings as Sabalenka lost serve again for 4-3 and her focus further unravelled.
At one point she dropped her racquet and kicked it across the court in frustration.
Rybakina, meanwhile, remained ice-cool and served out with the aid of two aces for her second major title.
Asked why she felt she lost momentum in the decider, Sabalenka said: “It was great tennis from her. Maybe not so smart for me.
“Today I’m a loser, maybe tomorrow I’m a winner, maybe again a loser. Let’s see.”