SAN FRANCISCO - Tesla's market share in its key California market tumbled in the first quarter of the year despite aggressive price cuts as rivals stepped up, data showed on Friday.

Tesla Inc controlled 59.6% of the battery electric market in California from January to March, down from 72.7% for the same period last year and the lowest since 2017, according to Reuters calculations based on data from the California Energy Commission.

Rivals such as Volkswagen AG, General Motors Co's Chevrolet and Kia Corp increased their market shares in California during the period, although they remain in the single digits each.

Tesla's sales in California accounted for 16% of the automaker's global deliveries last year, according to Reuters calculation. California is the biggest U.S. state for zero emission vehicles.

Tesla CEO Elon Musk's pursuit of Twitter and embrace of Republicans has sparked concerns about Tesla's brand, especially in liberal states like California.

Globally, Tesla posted record quarterly vehicle deliveries for the quarter, but quarter-on-quarter sales growth was modest despite price cuts as rising competition, especially in China, and a bleak economic outlook weighed.

The price cuts hit Tesla's first-quarter margins, pushing its shares down nearly 10% on Thursday.

(Reporting by Hyunjoo Jin; Editing by William Mallard)