The American came past her Serbian counterpart 6-4 6-4 at the end-of-season tournament in Singapore, where the year's top eight players do battle in two round-robin groups before the semi-finals and final.
After saving two break points in the first game of the event, Williams was the first to break today to move 4-1 ahead in the first set.
Ivanovic hit back by winning three games in a row to level it up at 4-4, but she couldn't move in front as Williams powered her way to the opener after moving 0-40 up on Ivanovic's serve at 5-4.
Ivanovic got off to a great start in the second set by breaking Williams in the first game, but the top seed hit back instantly.
The pair then shared eight holds of serve before Williams once again broke Ivanovic in the 10th game of the set to end her resistance.