"I don't care if I win, lose or break even," said Williams. "I'm ready to start (the U.S. Open), get it over with and be done and go on to the next event."
Halep had a solid start and broke Williams in the opening game of the first set but the American, who turns 34 next month, recovered to win the final five games of the first set.
In the final set, the opponents traded early breaks but Williams took it to another level in the tiebreak and wrapped the match up in 99 minutes.
"I tried to keep fighting," said Halep, now a loser in six of seven career matches against Williams. "It's not easy to return her serve. I had chances to break her, but she served really well in important moments."
Halep, who also lost the Toronto final last week, will take over second spot in the world rankings.
It marked the fifth title of the year for Williams following wins at the Australian Open, in Miami and at the French Open and Wimbledon.
The win was also the 69th career title for Williams and moved her ahead of Evonne Goolagong and into fifth place on the all-time list.
(Reporting by Tim Wharnsby in Toronto; Editing by Frank Pingue)
