backhand long at 15-40 to lose serve.
The Italian had won 13 of the first 19 points of the match.
However after a changeover courtside sit down; in the turning point of the match with Pennetta serving, Williams struck three groundstroke winners to reach 0-40 before breaking serve.
During her run of winning six games in a row to take the first set, Williams showcased some of her most powerful tennis of the tournament: hitting a 120 miles per hour ace down the tee and a 122 miles per hour unreturnable serve before striking a piercing crosscourt backhand winner to set up set point.
In set two, Pennetta held serve to open the set then later leveled the set at 2 games all.
Williams won the following three games to lead 5-2; striking a down the line backhand return winner to break serve.
Serving for the match, Williams hit a crosscourt backhand winner followed by a crosscourt forehand winner to lead 30-15.
Two Pennetta netted groundstrokes set up match point at 40-30.
The Italian netted a forehand to end the match after 63 minutes of play.
Today in the semifinals, Williams takes on the 17th seeded Ekaterina Makarova of Russia in semifinal number two.
Williams could win a record four million dollars if she wins the tournament based on her performance at the US Open plus the US Open Series warm-up tournaments.