The world number one has been battling with a knee injury in the latter stages of 2014 and she put in a mixed performance with 25 winners soured somewhat by 14 unforced errors.
Ivanovic put in a plucky showing in the opening match of the round-robin format that pits the top eight players in the world against one another.
However, in the end Williams' power was too much and she began her quest for a third straight Finals crown with a well-earned win in the Red Group.
The contest marked the fifth time the two have faced-off in 2014 and the early pattern of the match followed the form from their three most recent contests that were all won by Williams, when a rasping forehand brought the first break of the match and a 3-1 lead.
The American then followed with a hold to love, bringing up a 4-1 advantage with one of five aces she hit in the first set.
Ivanovic, whose only career victory over Williams came at the Australian Open in January, refused to lay down though and she broke to love when Williams double faulted when leading 4-2, before Ivanovic followed up with a hold to level the set.
US Open champion Williams then had to fend off another break point from Ivanovic to hold for 5-4, before she clinched the opener when she took the second of three set-point chances in the last game of the set.
A mistimed Williams forehand handed Ivanovic an immediate break at the start of the second set, but the Serb handed the advantage straight back.
The set then continued on serve with Ivanovic holding her own against the power of Williams.
But ultimately Williams found an extra gear and Ivanovic was unable to serve to stay in the match when trailing 5-4.
Speaking after the win, Williams paid tribute to the battling qualities of Ivanovic.
"I'm really excited with how I played tonight," she said. "It's super special. I couldn't be happier playing in Singapore.
"I was fighting really hard because Ana played really well, she played so good. I was just trying to fight as hard as I could."