Consistent baseline play from Errani coupled with 18 unforced errors
from Williams sent Errani up a set 6-0 after only 26 minutes.
In set two, hitting her serves and groundstrokes harder with more winners and decreased errors, Williams quickly took a 5-0 lead.
Serving to stay in the set, Errani hit a 66 miles per hour serve which Williams returned crosscourt on the forehand side for a winner.
Williams struck a second forehand winner and a backhand down the line winner before a Errani drop shot down the line landed wide to end the set.
Williams held serve to start set three while winning her seventh game in a row.
Errani then held serve to pull even at 1 all.
Williams double faulted then netted a forehand volley to fall behind 0-30 before losing a 24 shot rally at 15-40 after Errani struck a backhand down the line winner to lead 2-1.
The American then again improved her consistency midway through the set to break serve and lead 5-3.
Serving for the match, Williams was twice two points from winning the match; at 30-15 at which point she double faulted and at deuce.
At deuce, Williams netted a forehand down the line then struck a forehand down the line wide to lose serve to reach 5-4.
Errani held serve to reach 5 all after Williams struck several groundstrokes long.
After the American held serve to lead 6-5, Errani served to stay in the match.
The Italian went ahead 15-0 after Williams netted a crosscourt forehand. Williams then struck a swinging inside out down the line forehand volley winner to reach 15 all.
The American then netted a forehand and a backhand before hitting a crosscourt forehand return long as Errani would hold serve to reach 6 games all to enter the tiebreaker.
Williams, still one of the hardest hitters if not the hardest hitter on the WTA Tour, then faced a problematic situation which she often encounters in tiebreakers where each point is important.
Williams, who after the match said she went for too much, netted two crosscourt backhands as well as two down the line backhands for her 52nd unforced error of the match to trail early 2-5.
However a backhand down the line winner from Williams pulled the American to 3-5.
Errani struck a down the line forehand return wide to reach 5-4.
The Italian then struck a second serve with a speed excessively slow as the serve speed gun did not register it and remained blank. Williams took control of the point hitting a crosscourt backhand winner to level the tiebreaker at 5 all.
An unexpected crosscourt backhand volley winner from Errani set up match point at 6-5.
On match point, Williams struck a powerful backhand crosscourt before the 5 feet 5 inches tall Errani wrong footed the 6 feet 1 inch tall American with a surprise crosscourt forehand winner to end the match after two hours and one minute of play.
"Future's looking bright," Williams said. "You know, I definitely wish I could have taken it further today.There will be others. I think the more I play, the better I'll be."
Williams remains in the doubles competition at the US Open with her top ranked sister Serena Williams.