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.