Britain's Kyle Edmund suffered a surprise defeat by Canadian wildcard Steven Diez in the first round of the Rogers Cup in Toronto.
Diez, ranked 108 places below Edmund at 192 in the world, won 3-6 6-1 6-2 to claim his first win in the main draw of an ATP tournament.
Edmund, 21, was playing for the first time since winning both singles rubbers in Britain's Davis Cup win over Serbia.
Heather Watson and Naomi Broady open their campaigns later on Monday.
The women's event at the Rogers Cup takes place in Montreal.
Serena Williams, Andy Murray, Roger Federer and Rafael Nadal have all pulled out of the tournament, with Angelique Kerber and Novak Djokovic the top seeds.