The 32-year-old Aussie, who has had another injury interrupted year, was way off the pace in the first set but bounced back to win 6-1 6-7 (4-7) 3-6 in 2 hours 19 minutes.
Her ranking, which had slipped below 100 in recent weeks, is now certain to be comfortably into two digits.
Tomljanovic opened up a 4-1 lead in the second set, but even though she was pegged back she won the tiebreaker to force a deciding set against the Dane, who beat her at the Cincinnati Open in August in three tight sets.
Tauson was clearly in pain in the third set and took a medical timeout for treatment on her back. But at 5-3, Tomljanovic took advantage of her second match point to clinch victory.
Tomljanovic has now won three matches in four days in China, after progressing through qualifying. She will next face Turkey's Zeynep Sonmez, who beat Aliaksandra Sasnovich of Belarus in three sets.
Meanwhile, British No.1 Emma Raducanu again struggled physically in her first-round exit to China's Zhu Lin, slumping to a 3-6 6-4 6-1 defeat.
A week after retiring from her opening match in Wuhan with dizziness in hot and humid conditions, she looked lethargic early in the second set and called the doctor after seven games.
She had medical attention for her troublesome lower back and doubled over repeatedly before taking a medical timeout for treatment after three games of the third set.
Raducanu opted not to retire again but she was barely able to move and the end came quickly.
Elsewhere, Karolina Muchova beat Marketa Vondrousova in the all-Czech clash in straight sets.
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