World champion Lewis Hamilton and team-mate Heikki Kovalainen lagged behind their rivals in testing ahead of the Australian Grand Prix on 29 March.
But Dennis told BBC Sport: “Whatever performance level McLaren have today we will be a competitive racing team.
“That means we will be fighting for the world championship.”
While Ferrari have been consistently fast during pre-season testing, McLaren appear to be some way off the pace of their main rivals.
During this week’s test in Barcelona, McLaren‘s Kovalainen was more than two and a half seconds down on Nick Heidfeld’s fastest time for BMW Sauber on Monday.
Ferrari’s Kimi Raikkonen topped Tuesday’s timesheets, while another low-key showing for McLaren resulted in Kovalainen being 1.677 seconds adrift.
Istanbul Park Circuit Track Map

Istanbul Park Circuit Information
Circuit length: 5.338km
Race distance: 58 laps / 309.356km
Full throttle: 63%
Top speed: 315kph
Longest flat-out section: 16s / 1.2km
Right/left-hand turns: 6/8