Sagan beat star sprinters Andre Greipel of Germany and Caleb Ewan of Australia in a bunch sprint to win the 50.6 kilometer (31.4 mile) race over 22 laps of a street course in central Adelaide.
The win means Sagan will wear the tour leader's ocher jersey in the first stage of the six stage Tour Down Under on Tuesday. Sunday's race does not count toward general classification.
Ewan won the race in each of the past two years and Greipel is the only three-time winner. The 132-strong field that lined up for the race Sunday included seven former winners.
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