Short sellers taking aim at Tesla Inc. are holding onto bets that the electric car maker is on borrowed time, undeterred by the stock’s biggest one-day rally in years.

Heading into the electric car maker’s earnings report Wednesday, investors who had placed wagers on Tesla shares falling had racked up about $10.5 billion in short interest—making Tesla the most shorted stock in the U.S. on a dollar basis, according to financial-analytics firm S3 Partners. Short sellers borrow shares and then sell them, hoping to buy back the...