Nick Heidfeld has admitted that his Formula One season was only 'so-so' after he struggled during the middle of the 2008 campaign.

The German started the year with four top-six finishes in five races but as the season progressed, he proved to be unable to match the pace of team-mate Robert Kubica.

Despite second place finishes in Canada and Britain, poor performances in Monaco, France, Hungary and Valencia - the latter being a race he described as one of his worst ever - led to speculation that Heidfeld could be replaced for 2009 although BMW Sauber later confirmed that it will field an unchanged line-up next season.

Heidfeld admitted however that his performances during the middle of the season hadn't been up to scratch but said he was pleased with the way he had rebounded from his problems too score points in four of the final six races.

"So-so, with an upwards curve towards the end of the season," he said when asked to sum up 2008. "It is always my aim to get the maximum from the car and I didn't manage to do that through the middle part of the season - in qualifying in particular.

"The positive thing is that I came through this difficult period by analysing the situation, rather than stubbornly ploughing on. Highlights included the race at Spa - when my decision to change onto wet-weather tyres two laps from the finish paid off - and five passing manoeuvres over the course of the season in which I overtook two cars in the same move. And, of course, four second places aren't bad going either."

Having admitted to a struggle during 2008, Heidfeld added that he was hopeful that the reintroduction of slick tyres into F1 next season would work to his advantage, while he also admitted he expected to see more overtaking on the grid in 2009.

"We'll find out in due course if slicks suit my driving style specifically, but I believe they will do and I definitely think it's important that we get them back in Formula One!" he said. "Slicks are simply part of F1, both for the sport as a spectacle and as far as driveability is concerned.

"Anything else is just a joke. In combination with the significant reduction in the cars' downforce levels, slicks will hopefully also result in more overtaking."