Williams heads into the US Open trying to become the first player since Steffi Graf in 1988 to pull off a calendar Grand Slam sweep.
"I'm ready," Williams said. "I don't care if I win or lose or break even. I'm ready to start it, get it over with and be done and go on to the next event. But I'm so ready for New York."
Halep was the runner-up at Toronto a week earlier and was hoping to beat Williams for the second time in less than a year after upsetting her at the WTA Finals in October. Halep broke Williams' serve to open the match but couldn't sustain it.
Williams won four of the last six points in the tiebreaker, winning the match when Halep sliced a backhand shot into the net.
Then the talk about the Grand Slam began.
"I know you can do four," Halep told Williams during the trophy presentation.
Serena defends Cincy title
Serena Williams defeats Simona Halep 6-3, 7-6 (5) to become the first woman to claim back-to-back Cincinnati titles.
Serena: 'It really wasn't easy'
Serena Williams talks about her two-set victory over Simona Halep to become the first woman to successfully defend the Western & Southern Open title.