By Rob Copeland 

Bill Gross's decadeslong run as one of the world's top fund managers started by accident--and with one.

Bedridden at Duke University Hospital in 1966 after a car crash, Mr. Gross studied a book about counting cards. He healed and headed to Las Vegas, turning a $200 investment at the blackjack tables into $10,000.

"I wanted to prove you could beat the system," he later told The Wall Street Journal. "I thought about what I could do that takes the same skills. I realized it was investing."

Nearly half a century after founding Pacific Investment Management Co. and building it into the world's largest bond-fund manager, Mr. Gross is out to prove himself again.

Mr. Gross, 70 years old, stunned Wall Street on Friday by resigning from his Newport Beach, Calif., firm after months of tension with his colleagues and amid concerns among investors that led to massive outflows from his flagship Pimco Total Return Fund.

He will join the far-smaller rival Janus Capital Group Inc. on Monday. Mr. Gross, and Janus, are betting that his star power and his skills as an investor will help lure money to Janus from Pimco and others.

Mr. Gross earned a name as the "Bond King" with his knack for predicting well ahead of the pack where interest rates were headed.

And long before it was fashionable on Wall Street to eschew a button-down approach, Mr. Gross built a brand steeped in his loose, Californian style.

As money flowed into Pimco from corporate pensions, sovereign-wealth funds and individual investors of all stripes, Mr. Gross stuck to his image as an open-collared, yoga-loving, stamp collector.

He won big in 1995 with a wager that long-term interest rates would fall, and roiled the bond market in 2000 by dumping mortgage securities and buying up long-term Treasury bonds.

His moves spurred copycats on Wall Street trading desks.

"Once it was confirmed that Bill Gross was doing this, everyone seemed to turn back to their portfolios, scratch their head and take another look at their own allocations," one money manager told The Wall Street Journal in 1996 after Mr. Gross put $2 billion into Treasurys.

When he was wrong, he admitted it. He was early to predict lower Federal Reserve interest rates in response to a slowdown in U.S. housing, a call that cost him in 2006 as rates stayed high. Mr. Gross conceded it was a "big mistake."

While other asset managers spent lavishly on advertising to get their mutual funds in front of investors, Mr. Gross became a fixture on business television for more than two decades, helping to spread the Pimco name.

Along the way, Mr. Gross became a favorite of investors and journalists for his colorful takes on the once-sleepy fixed-income market.

"We've shot the rapids, gone over the waterfall," he told clients after a boom in risky bonds in 1993. He compared credit-ratings firms' generous bond ratings in 2007 to "six-inch hooker heels, and a tramp stamp."

But those idiosyncrasies came with an edge. A former naval officer in the Vietnam War, Mr. Gross showed flashes of temper.

He scolded employees and quarreled with his heir apparent, Mohamed El-Erian, who resigned in January, as well as with a host of new deputies.

Mr. Gross, who was due to be fired from Pimco before he tendered his resignation, said in a statement Friday he was happy to be "giving up many of the complexities that go with managing a large, complicated organization."

Sydney Finkelstein, a management professor at Dartmouth College's Tuck School of Business, said: "It's not clear that his skill set is as a leader or a manager. It's as a genius investor."

Melissa Korn contributed to this article.

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