(Reuters) - Tesla Inc TSLA.O shares soared 17% on Thursday after the electric carmaker surprised Wall Street by delivering on Chief Executive Officer Elon Musk's promise of a profit in the third quarter, even as doubts remained about its long-term prospects.

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Trading at $298, Tesla's market capitalization was $53 billion, surpassing General Motors Co's GM.N $51 billion stock market value and making it the United States' most valuable car company. It has been the No. 1 U.S. car company by market cap before, but recently GM has had a substantial lead.

Tesla late on Wednesday reported a quarterly profit, citing improvements in operating efficiency and a reduction in manufacturing and material costs.

The strong report unleashed a bloodbath on traders shorting Tesla, the second most shorted U.S. company, after Apple Inc AAPL.O, in terms of the overall amount of money shorted.

With $10.5 billion bet against Tesla, short sellers suffered paper losses of $1.4 billion on Thursday, erasing 70% of the profits they had logged in 2019, according to S3 Partners, a financial analytics firm. Tesla’s shares remain down 10% year to date.

Tesla’s $1.8 billion junk bond due in August 2025 surged 3 points in price following the results, driving its yield to the lowest since March 2018.

“A strong step forward, yet Tesla will need to put together a string of similar data points to demonstrate the sustainability of results ... and its track record has been spotty on this,” said analysts at brokerage Credit Suisse.

At least eight brokerages raised their price targets on Tesla shares, while the company’s average rating on Wall Street remained “hold”, with just 11 of 34 analysts recommending investors buy the stock.

Investors in the past have shown impatience with the company’s serial failures to meet financial and production targets and shares in the company are still down more than a third from their 2018 peak of almost $390.

The company has been struggling with margins and a new Chinese factory is expected to be help on that front, but analysts said the jury was still out on whether it can sustain the push into profitability.

Click here tmsnrt.rs/360v0Y4 to see charts on Tesla's margins and deliveries.