Williams, who had previously lifted the Daphne Akhurst trophy in 2003, 2005, 2007 and 2009, wrapped up the win with a forehand winner after two hours and seven minutes to become the first women since Jennifer Capriati in 2002 to defend the title in Melbourne.
The victory secured Williams’s 12th Grand Slam title, leaving her equal with compatriot Billie Jean King in sixth place on the list of female Grand Slam champions.
Starting the match in 35 degree heat, even though it was past 7.30pm in Melbourne, Williams looked nervous as she struggled to hold serve despite opening the match with an ace.
Williams was forced to save two break points in the first game before being allowed to go on and hold after eight minutes when Henin sent a nervy forehand long of the line.
Henin found things slightly more comfortable on serve, getting her account off the mark with a backhand winner that delighted the crowd, before again earning herself break points against the Williams serve.
Almost from nowhere, Williams bounced back from her troubled start and looked a different player when she broke to love in the fourth game as Henin sent an off-balanced backhand wide down the line.
Henin, who won the Australian Open title in 2004, enjoyed break back points in the very next game but could not capitalise and was forced to wait until the seventh game before getting things back on level terms, whipping a cross court forehand passing shot away to seal the break.
Williams bounced back immediately though, wrapping up the first set after 51 minutes when Henin’s cross court backhand hit the top of the tape and bounced wide.
The second set was similarly competitive with Henin holding a multitude of break points, the Belgian breaking in the third game with a cross court forehand winner off a poor drop shot.
Williams continued to frustrate Henin in the early stages of the second set, breaking back immediately before holding to take the lead at 3-2.
But Henin suddenly found another level, reeling off four games in succession, breaking in the seventh game with a huge forehand that Williams could only return long, before sealing the set when Williams dumped a forehand into the net.
Henin looked in danger of running away with the early stages of the final set as she held to love in the opening game and set up two more break points in the second.
But Williams regained her composure, forcing her back into the match and even trading breaks with Henin before running away with the last four games to pick up her fifth Australian Open with a forehand winner across court.
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Pippa Davis / Eurosport