"I've always been a good player off the jump,'' Williams said. "That's kind of always been the way I've played throughout my career. I never played a lot of big warm-up events. Coming to tournaments, I've always been able to play well when I entered.''
Williams served and hit powerfully, while Jankovic appeared sluggish, her serves lacking pace and her groundstrokes erratic.
Jankovic said she is usually a slow starter and is getting used to her bulkier body following an offseason fitness training program.
"I need more time to adjust, to get my rhythm and feel the court, feel the ball,'' Jankovic said. "You could see, I was very, very slow out there. My reactions were quite slow.''
Williams also beat Jankovic in the semifinals of the season-ending WTA Championships in Doha, Qatar, on her way to taking the title.
The American broke in the first and third games to go up 4-0 in the first set. Jankovic squandered four break points in the sixth game to fall 5-1 behind, and Williams served two aces and hit a down-the-line forehand winner to close out the set.
Jankovic dropped serve in the third game of the second set to trail 2-1. She missed a forehand in the seventh game to give Williams a second break and a 5-2 lead. Williams served an ace and two other serves forced errors in the next game to clinch victory.
The team-format event in Hong Kong is both players' only preparation before the Australian Open starts Jan. 19 in Melbourne.
Williams' win tied Team Americas with Team Europe at 1-1.
Later Tuesday, Argentina's Gisela Dulko beat Hungary's Agnes Szavay 6-4, 6-7 (8) 6-3 to help Team Americas win the tie 2-1.
Team Americas will next play Team Russia, which beat Team Asia Pacific on Wednesday after two straight-set victories by Vera Zvonareva and Anna Chakvedatze.
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