Williams, 30, was barely troubled by 10th-seeded Errani, the French Open runner-up, who she had already defeated three times in three outings prior to Friday's mismatch.
She broke three times in the first set, which was wrapped-up in 30 minutes, firing 18 winners to just three and allowing the Italian just five points on her serve.
More breaks followed in the first and fifth games of the second set as the American built a 4-1 lead.
Errani, the first Italian woman to reach the US Open semi-finals, saved a match point in the seventh game.
But Williams, the runner-up to Australia's Samantha Stosur in a bad-tempered final in 2011, wasn't to be denied, winning the tie with a second serve ace - her ninth of the contest and 50th of the tournament.
Her 64-minute victory was perfectly illustrated by 38 winners to just six by Errani.
-AFP