Williams has now won 27 of her last 28 matches, including victories at Wimbledon, the Olympics and the US Open, with the only blemish a reversal to Kerber in the quarter-finals at Cincinnati.
Williams told wtatour.com: "Obviously, I'm thinking, 'Okay, she's beaten me before, so she is going to be confident she can beat me again. What can I do to improve?'
"But I really just wanted to do well and win today [Tuesday]."
In Group B, second seed Maria Sharapova started her campaign with an equally convincing 6-3 6-2 win over French Open finalist Sara Errani of Italy, breaking her opponent twice in each set.
Also in Group B, Poland's fourth seed Agnieszka Radwanska got off to a winning start against reigning champion Petra Kvitova, coming through 6-2 6-3 in one hour and 23 minutes.