The Romanian was a 6-3 2-6 6-3 victor, dropping a set for only the second time in the week, but standing firm in the clash that clocked in at over two hours.
That meant records for Haddad Maia, who has played the most WTA-level matches with three sets in 2022 (20) and spent over 12-and-a-half hours on the court at the Canada Open in 2022, more than any other player in a single WTA tournament this year.
Wimbledon semi-finalist Halep proved to be a step too far for the Brazilian, however, with the win marking her most significant honour since winning in Rome in 2020.
It was far from a vintage performance from the 30-year-old, who had nine double-faults in the match compared to Haddad Maia's two and left the door open for the South American - who was vying for victory in her first ever WTA 1000 event.
Halep had already clinched a return to the world top 10 by reaching the final in Toronto, the first time since her 373-week stint ended just over a week ago - which led to admissions that she considered calling it quits amid her decline.
A valiant comeback in 2022 has been one of the major stories of the calendar year though and, on the back of victory in Canada and a semi-final march at Wimbledon, will have high hopes for the U.S. Open.