Daimler AG is expected to book a $780 million windfall from the sale of its four percent stake in U.S. electric car maker Tesla Motors.

The move, which occurred on Tuesday, surprised investors and fueled speculation of a rivalry. The maker of Mercedes-Benz cars invested in Tesla in 2009 as a way of obtaining cutting edge battery technology.

Tesla provided technology for the electric version of Mercedes Benz B-class cars and Smart vehicles, according to Reuters.

Daimler has said that cooperation deal remains "intact," arguing that shareholding wasn't necessary for it to continue working with the California-based company.

Tesla has not commented on the sale or future collaboration yet.

Analysts said the sale could be read as a sign that the once-close partners now view each other as rivals in the competitive electric car market.

"There is a difference between working together and partnership. In our view the ties between the two companies have loosened over time," analysts at ISI Research said in a note, according to Reuters.

Daimler acquired a 9.1 percent stake in Tesla for approximately $50 million back in May 2009. Due to capital increases, Daimler's stake will decrease eventually to approximately 4 percent.

Tesla was still a small company at the time of the initial investment, and the shareholding provided the German company with preferential access to the U.S. electric car marker's technology, according to Reuters.

Back in June however, Tesla said it would allow other rival companies to use most of its patents in hope of advancing investments in electric vehicle technology. This decision might have made an exclusive partnership seem less attractive for Daimler.

Demand for U.S. premium eco-cars has steadily risen since 2009. In the U.S., the Tesla Model S outsold the old version of the Mercedes-Benz S-Class last year, analysts at ISI said.

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