Coco Gauff broke Katie Boulter's serve five times on the way to triumphing in one hour and 13 minutesBritish number one Katie Boulter was swept aside in straight sets by American teenager Coco Gauff in the second round of the National Bank Open.
Boulter, 27, had not dropped a set in coming through two qualifying matches and her main-draw opener in Montreal.
But she lost 6-2 6-2 to 19-year-old Gauff, who will face Wimbledon champion Marketa Vondrousova or the returning Caroline Wozniacki in the last 16.
Ex-world number one Wozniacki won her first-round comeback match on Tuesday.
Gauff, ranked sixth in the world, made a blistering start against Boulter on Wednesday as she lost just three points in the first four games of the opening set, twice breaking serve to establish a 4-0 lead.
World number 72 Boulter responded with a break of her own and later saved two set points at 5-2 and 40-0 down but Gauff converted the third.
The Briton failed to take a break point at the beginning of the second set and was broken three games later before Gauff sealed victory with another break, despite Boulter saving three match points.
Elsewhere, fifth seed Caroline Garcia, of France was upset 6-4 4-6 6-2 by the Czech Republic's Marie Bouzkova and Sloane Stephens enjoyed a walkover into round three after Victoria Azarenka's withdrawal through injury.
The world's top three women - Iga Swiatek, Aryna Sabalenka and Elena Rybakina - are among the players in action later on Wednesday, while Wozanicki continues her comeback by facing Vondrousova at around 21:30 BST.
Rybakina will resume in a first-set tie-break against American Jennifer Brady after their second-round match was halted by rain on Tuesday night.
