The top-seeded American broke the eighth-seeded Serb twice in the first set and three times in the second to register her 73rd win of the season.
Williams appeared in pain after serving an ace during the third game of the second set and her serve was broken for the only time in the match in that game as Jankovic went 2-1 ahead.
But it was Jankovic who had to take a medical time out after the game due to a problem in her hip.
The Serb, however, failed to win another game upon her return to the court as Williams racked up five games in a row to close out the match.
"I'm really excited. I didn't think I'd win this tournament starting out, so I'm really happy to be here holding the trophy," Williams, who complained of a sore back after Saturday's semi-final, said during the presentation ceremony.
Sunday's victory also gave the 32-year-old Williams her 56th career title.
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