The world number one had barely played any competitive tennis since her U.S. Open exit last September and entered a sweltering Rod Laver Arena with doubts over her fitness to face the highest-ranked unseeded player in the draw.
The slight but hard-hitting Giorgi conceded a 4-1 lead early but steadied to give the American a genuine test and Williams' roars of frustration underlined the growing tension as the match progressed.
However, it was Giorgi who wilted at 5-5 in the second set, a double-fault handing Williams the decisive break and allowing the American to close out the match with a barrage of booming serves.
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(Reporting by Ian Ransom; Editing by Peter Rutherford)
