BMW Sauber team principal is certain that the collision between Sebastian Vettel and Robert Kubica three laps before the end of the Australian Grand Prix cost the Pole his second victory in Formula 1.
The pair made contact as Kubica tried to pass Vettel for second position on the outside of Turn 4, having gained ground dramatically on the Red Bull as well as leader Jenson Button - both of whom were running on the softer slick tyres.
Kubica and Vettel, former BMW teammates, subsequently crashed into the barriers further on in the lap because of damage sustained in the initial impact.
But such was Kubica's late-race pace, that Theissen told the BBC that Vettel's failure to yield the corner cost BMW dearly: "Sebastian braked early and I thought: 'This is a clear-cut situation now, I know him, he won't do in any silly things.' But in this moment it had happened already.
"It's been very disappointing for both drivers, we are very confident Robert had won the race because he had closed in on the hard tyres," Theissen added.
"Both cars in front of him were struggling on the soft tyres. He was lapping about two seconds quicker and he lost probably a victory, Sebastian lost probably a podium position and both teams lost valuable points."
Brawn GP and Button dominated for much of the race, but while Theissen accepted Button and team-mate Rubens Barrichello had a performance advantage over BMW in Melbourne, he believes the severe drop-off on the soft tyres at the end of the race gave Kubica superior pace.
"The Brawns definitely had performance in hand on the car side, but they were struggling with the soft tyres as well," he said.
"The upside is that the car was really quick. Robert was putting in very quick laps on the hard tyres and that shows that we are on the pace which is good for the coming races."