Mercedes-GP are facing a "huge challenge" to maintain their 100 per cent winning record in Formula One in 2019 at Sunday's Canadian Grand Prix. That is the view of the German marque's team principal Toto Wolff despite his team having triumphed in the opening six races of the campaign. Wolff believes the layout of the Circuit Gilles Villeneuve track in Montreal could suit Ferrari, who have been their nearest challengers the majority of the time this year. Sebastian Vettel won from pole position in Canada last year for the Italian team and Mercedes have suggested the high-speed circuit could again favour them. "We are beginning to see more clearly the strengths and weaknesses of our car. In the past six races, we were very strong in the corners but lost time on the straights," said Wolff. "This will make Canada a huge challenge for us as the track characteristics could favour our opponents - there are many long straights, and fewer corners in which to make up lap time." Montreal has been a happy hunting ground, however, for championship leader Lewis Hamilton in the past. He has been victorious six times there, most recently in 2017, and if he prevails again in the 70-lap race on Sunday, which begins at 10.10pm UAE time, he will equal Michael Schumacher's record of seven wins at the venue. Hamilton, who took the first Formula One win of his career at the circuit with McLaren in 2007, indicated after winning the previous race in Monaco that a Mercedes engine upgrade was expected. While Mercedes have kept a lid on their plans, the timing would be right since the rules allow each driver three engines per season and Canada comes a third of the way through the 21-race championship. Ferrari, Renault and Honda have already introduced their upgraded engines. A main concern for the champions will be tyre management, with Hamilton struggling on the wrong choice in Monaco and being fortunate they were at a a track where overtaking is nigh on impossible. "We need to learn from Monaco ... because this situation in Montreal, you lose the race," Wolff told reporters at the time, cautioning also against too much confidence after such an unprecedented start to the campaign. "We must never be carried away by saying we are just flying at the moment and it's going to continue like this," said the Austrian. Hamilton leads teammate Valtteri Bottas by 17 points at the top of the standings, while the highest non-Mercedes driver, Vettel, is already a distant 55 adrift. Ferrari's team boss Mattia Binotto is less convinced that one of his cars will win a race for the first time since October. "We know we're not competitive enough right now," he said. "And for the time being we haven't got any more changes coming on the car that will have a significant effect on the problems we have encountered since the start of the season."