Canadian Vasek Pospisil powered to a 5-7, 7-6 (7/5), 7-6 (7/4) victory over John Isner to send the big American back across the border on Tuesday.
Pospisil, the ATP number 71 who claimed a Challenger title at the weekend in his native Vacouver, led a home charge in the ATP Masters event on the Montreal hardcourts which also saw wild cards Filip Peliwo and Frank Dancevic reach the second round.
They joined Jesse Levine, who advanced on Monday.
Canadian number one Milos Raonic, the 13th seed struggling to snap a losing streak, was next up on the home courts, facing Frenchman Jeremy Chardy in the opening round.
Top seeded Serbian Novak Djokovic, winner of the last two Canadian editions, was also in action on Tuesday, beginning his campaign for a hat-trick against Germany's Florian Mayer.
Pospisil's victory, in two and a half hours, gave him a 2-1 career lead over Isner, who was the lone American entry this week.
Pospisil fought back after dropping the opening set in what was looking like a routine hardcourt win for Isner, who was runner-up on Sunday in Washington to Argentine Juan Martin Del Potro and Atlanta champion the week before.
The 23-year-old Canadian began to make an impression as he levelled with a tiebereak second set and came from a break down in the third to square it at 4-4 as Isner slammed an approach shot into the top of the net.
With the crowd behind him and his game on fire, Pospisil scrambled into a tiebreaker only to lose four straight points as Isner recovered from 0-2 down.
But Pospisil rallied with a return at Isner's feet for a 5-4 lead and earned two match points with a reflex volley winner.
Isner, a semi-finalist in this tournament last year, went down on the first match point.
"This is such a win, I really believed in myself today," said Pospisil. "It was an extremely tough match, especially with his huge serve. But I had confidence from Vancouver."
The Canadian next plays Czech Radek Stepanek, who upset Spanish 12th seed Nicolas Almagro 6-3, 6-7 (4-7), 6-3.
In the other Canadian matches, 2012 double junior Grand Slam winner Peliwo won his his Montreal senior debut saving a match point in the second set and advancing as Finn Jarkko Nieminen retired with hamstring trouble while trailing 3-6, 7-5, 3-1.
"Especially being down two breaks, and match point, I thought the match was pretty much over," said Peliwo. "I don't really know what I did, it's definitely the best feeling I've had in a while.
"I have confidence after beating top 50, top 40 player, I'm still trying to realise what I did."
Dancevic also had to rally for a 5-7, 7-6 (8/6), 6-1 victory over Taiwan's Lu Yen-hsun.
Italy's 13th seed Fabio Fognini, winner of two titles and 13 of 14 post-Wimbledon matches on clay, made a winning transition to hardcourts with a 1--6, 6-1, 6-1 win over Marcos Baghdatis.
Polish 15th seed and Wimbledon semi-finalist Jerzy Janowicz also needed a comeback to overcome France's Julien Benneteau 3-6, 6-3, 7-5.
Russian Alex Bogomolov notched a 6-2, 4-6, 6-3 win over Frenchman Michael Llodra, who was hindered by an injured elbow, while Russian Mikhail Youzhny opened with a 6-4, 7-6 (7/3) win over Jurgen Melzer.