Theissen still wary of Heidfeld's form

Heidfeld had qualifying struggles at the start of the year but a mid-season breakthrough has transformed his campaign and left him as the highest scoring driver in the last four races.

But despite the improvement in form making it increasingly likely he will be retained alongside Robert Kubica next year, Theissen wants to see more before he is convinced Heidfeld is back to his best.

When asked by autosport.com if he was surprised how quickly Heidfeld has lift his game, Theissen said: "I had hoped for it, but I still want to see some more races.

"I am very confident that he has overcome the weak period, but you don't get into this within a week and you don't get out of it within a week.

"So, he has to work systematically - and he is working systematically. And I am confident he will get back to full performance and full confidence."

BMW Sauber are believed to have options on both their drivers for next year, with a decision on their plans due to be made before the clauses expire in September.

Theissen said, however, he was unsure when he would make a final decision on his plans.

"I cannot give you an exact date," he said.