Ferrari have beaten Mercedes three times so far this year

Kimi Raikkonen says Ferrari have "surprised" themselves with their performances at some races this season, although the Finn admits the remaining weakness on their car won't all be solved this year.

With five races of the season left - starting with the Russian GP in Sochi this weekend - Ferrari sit a comfortable second in the Constructors' Championship, two places and 159 points better off than they were at this stage of their winless 2014 season.

The SF15-T has generally been the closest challenger to the Mercedes W06 in 2015, winning on three occasions, but Ferrari have lagged further behind their rivals at certain tracks, particularly those with high-speed corners such as Barcelona and Silverstone.

Raikkonen acknowledges that Ferrari still don't have a car for all circuits, although he does believe they have continued to make good progress since the start of the campaign - as shown at Suzuka, a track which didn't automatically suit their car but where Raikkonen and Sebastian Vettel were only beaten by Mercedes.

"Certain tracks are trickier for our car depending on the layout right now. We know when it's going to be a bit trickier, but we have surprised ourselves," the 2007 world champion said.

"We expected Monza to be a bit more difficult, we were quite good there. [Suzuka] wasn't too bad even though it shouldn't be our favourite place right now. But it shows that we are doing the right things and improving things."

Ferrari team-mates Vettel and Raikkonen have enjoyed a successful first season in tandem

The Finn, however, concedes that additional progress over the final seven weeks of the season can only be relatively modest.

"We know where the weaknesses are but there are limited things that we can do at this time of the year," he added. "I'm sure we will keep improving little by little and next year we'll be in better shape again."

Although on course to beat Williams to second place in the standings this year, Ferrari's mathematical chances of catching Mercedes will officially end on Sunday if the reigning world champions outscore them by three points.

Mercedes clinched last year's Constructors' Championship - the first in their history - in Sochi last year.

Don't miss Sky Sports F1's live weekend-long coverage of the 2015 Russian Grand Prix. Our race-day show begins at 10.30am on Sunday, with lights out at 12 noon. Watch the Russian GP for £6.99 with NOW TV