Andy Murray's preparations for the US Open were dealt a blow with a 6-3 6-1 loss to world number 35 Kevin Anderson.
Murray, the defending champion, was playing his first match at this year's Rogers Cup in Montreal having been handed a bye into the second round.
But the Scot looked rusty as his 6ft 8in opponent from South Africa dominated from the net, breaking Murray's serve three times.
"I just felt very slow, the game seemed to be going so fast," said Murray.
The Briton, ranked number four in the world, served five double faults and looked sluggish in comparison to Anderson, who never allowed Murray to settle.
"I've trained really hard to get ready for the tournament. I've always played very well here," Murray, who has won the last two Masters titles in Canada, told BBC 5 live.
"I couldn't get anything going. I started both sets really, really badly which doesn't help against someone that serves like Kevin.
"I trained in Miami for two weeks before I came here on Thursday," added Murray.
Anderson was ruthless as he held for a 3-0 first-set lead before Murray eventually managed to hold in the fourth game of the match in blustery but warm conditions in Canada.
But a lacklustre Murray had no answer as Anderson, growing in confidence with every game, sealed the first set with an ace.
A series of unforced errors crept into Murray's game in the second set and Anderson ran away with it, condemning Murray to a defeat that is far from ideal preparation for the final major of the year, which gets underway in New York in less than three weeks.
"I came to the net a lot more and won a lot of points," said Anderson. "I'm playing really good tennis at the moment."
Murray, whose loss clears Rafael Nadal's path to the final, will now look to regroup in the doubles with brother Jamie and will hope to bounce back in the singles in Cincinnati next week.
In the day's other games in Montreal, home favourite Vasek Pospisil shocked Argentina's Juan Ignacio Chela 4-6 6-3 6-4 to set up a second-round meeting with third seed Roger Federer.
Serbia's Janko Tipsarevic beat Colombia's Alejandro Falla 7-5 6-1, but world number one Novak Djokovic was unable to get his campaign off and running after rain brought an early end to the day's play.
Djokovic was due to play Nikolay Davydenko in what is his first official match since moving to the top of the rankings after winning Wimbledon, but the match will now be played on Wednesday.