The 24-year-old Swede broke to lead 3-1 in the first set before Williams fought back to 3-3. Larsson advanced to 4-3 before caving in to the American's strong serve and losing the next three games.
Williams dominated the second set, breaking Larsson's serve twice.
"It was really good to come here and win after a disappointing Wimbledon so hopefully this confidence will help me for the rest of the year," said Williams, who lost in the fourth round at Wimbledon to Sabine Lisicki.
Larsson said "at the crucial points I wasn't really present, which was a shame."
Williams also defeated 76th-ranked Larsson in their only previous match in the Fed Cup earlier this year.
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