A year ago, the American was playing Toronto ranked No.934 in the world, and another runner-up finish to the world's top ranked player kicks off her US Open title defense in encouraging fashion.
Stephens rallied to within 5-4 before Halep, on her fourth match point, slammed her third ace of the match and dropped to her knees in triumph.
Once again though Stephens appeared to be on her way to squandering the momentum, wasting three set points in the ninth game before finally converting her to force a decisive third set. Halep battled back in the next game, holding two set points from Stephens, to set up a tiebreak.
Halep made the stronger start, dictating rallies from the baseline to race into a 4-1 lead after twice breaking Stephens in the opening five games. I thought she played well.
The 26-year-old also won when the event was last held in Montreal in 2016.
Halep broke serve in the third game and, although Stephens forced a break point in the next, she could not capitalise.
"It was really tricky conditions. we were both struggling a little", said Barty, who never faced a breakpoint during the lopsided encounter. She contended she has the worst schedule in the 56-player event and "this happens nearly every tournament".
"I think I played smart tennis today", Halep said.
Stephens led throughout the second set, breaking Halep twice, but the Romanian took control of the third set, turning a break into a 5-2 advantage. Halep has won her past six meetings with the 25-year-old American. It was the first set Stephens lost all week.
"I was very sore when I woke up", Halep said.
Stephens again looked to be on the brink of the first set after surging into a 5-1 lead which soon became 6-4 in the tiebreak.
Both players traded early breaks in the decider before Halep secured the 13th break of the match in the fifth game for a 3-2 lead.
Halep was not to be denied however, thumping down an ace to seal a memorable win on her next service game. However, there were just as many pulling for Stephens in a match of two players who don't overpower with their serve but hammer groundstrokes down the lines. Stephens fought off three match points for one final service break before Halep closed it out. "I hope in the future it's going to be better".