The world number one claimed her second straight Cincinnati trophy and moved past Aussie legend Evonne Goolagong into fifth place in the all-time WTA win list in the Open Era. Martina Navratilova heads the list with 167 singles crowns.
The 33-year-old American rallied past third seed Halep 6-3, 7-6 (7/5) on Sunday for her fifth title of 2015.
"I think that playing Simona really tested me, and I felt like I was up for the challenge and up for the test," Williams said.
"I definitely feel there is room for improvement."
Halep, who handed Williams one of the worst losses of her career at the 2014 WTA Finals in Singapore, will surpass Maria Sharapova as the second ranked player in the world when the new rankings come out Monday.
Williams will be bidding for history at the US Open as she attempts to complete a rare calendar-year Grand Slam in singles. She came back from an early first-set deficit to take charge against Halep.
Williams is looking forward to the start of the US Open in eight days.
"I prefer to have this pressure than the pressure of not winning," she said.
"Not everyone can handle that pressure, but I'm OK with it. I would rather be in this position than another one."
Williams went down an early break but reclaimed it and then some to win the opening set. The second was much more combative as Halep held her own from the baseline.
But Williams turned up the heat in the tiebreaker to finish the job in 99 minutes with 15 aces.
Williams now stands 21-4 in Cincinnati as she moves on to New York for training. The American has won all five of her 2015 finals - three Grand Slams, a Miami title and now Cincinnati.
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