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.
More Sports Headlines
Lions' Tate returns to CenturyLink Field to face his former team Lions’ TE Ebron could cause problems for Seahawks Seahawks' secondary won’t be bored Monday night against Lions Mariners end season with 3-2 victory over A's Seahawks' Lynch, Mebane won't play Monday night against Lions An open letter to new M's GM Jerry Dipoto Barrett scores in stoppage time, Sounders tie Galaxy 1-1 Gurley rushes for 146 yards, Rams hang on to beat Cardinals 24-22
Our new comment system is not supported in IE 7. Please upgrade your browser here.