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
Snohomish mixed martial artist returns to her first love Mariners beat Twins 4-3 in 11 innings Charles scores 29 points, Liberty beat Storm 78-62 Park wins Women's British Open, captures 4th different major Perry wins 2nd straight Champions Tour's 3M Championship Merritt shoots 67 to win Quicken Loans National by 3 shots Triple Crown winner American Pharoah wins $1.75 million Haskell Invitational Bills' Ryan names Incognito starting left guard
Our new comment system is not supported in IE 7. Please upgrade your browser here.