Third seed Dominic Thiem has squandered four match points in a shock second-round loss to Diego Schwartzman at the Montreal Masters.
The world No.7 had battled back from a set down to lead 5-2 in the decider but lost five games in a row against the Argentinian, who prevailed 6-4 6-7 (7-9) 7-5.
In the day's other second-round contest, 20-year-old Jared Donaldson had few problems in overcoming Frenchman Benoit Paire 6-2 7-5.
Donaldson's fellow Americans Jack Sock and Wimbledon semi-finalist Sam Querrey also saw off French opponents, with Querrey requiring three sets to oust Vincent Millot 4-6 7-6 (7-4) 7-5.
Querrey's frustration boiled over early in the third set when he broke his racquet after having his service broken.
He went on to win the next four games after the outburst.
Querrey goes on to face another Frenchman, Jo-Wilfried Tsonga, the 2014 Rogers Cup champion in the round of 32.
Sock, the 15th seed, beat Pierre-Hugues Herbert 7-6 (7-4) 6-3.
The home crowd had something to celebrate when 18-year-old wildcard Denis Shapovalov overcame a difference of 79 places in the world rankings and saved four match points in his 4-6 7-6 (10-8) 6-4 victory against Rogerio Dutra Silva.
Ninth seed David Goffin came from a set down to beat Yuichi Sugita 4-6 6-1 6-4, while 11th seed Pablo Carreno Busta also came through in three sets against Karen Khachanov.
Paolo Lorenzi saw off Frances Tiafoe in three sets to tee up a second-round meeting with Nick Kyrgios, but world No.28 Feliciano Lopez was shocked by Hyeon Chung, who clinched a deciding tiebreak.
Kevin Anderson, Borna Coric and Ernesto Escobedo also won their openers on Tuesday.
But 10th-seeded Czech Tomas Berdych withdrew from the tournament before his first-round match against Nikoloz Basilashvili.
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