By Svea Herbst-Bayliss

BOSTON, July 31 (Reuters) - Hedge fund mogul David Einhorn's Greenlight Capital slumped 6.1 percent in July and is now down 9 percent for the year after gold, one of the fund's top holdings, tumbled to five-year lows last week.

Greenlight notified clients of its returns late on Friday, according to one source who shared the numbers with Reuters.

Einhorn, who owns physical gold and called it one of his biggest bets in this month's investor letter, is now one of the first prominent fund managers to show just how deeply this month's gold rout has weighed on performance.

Greenlight, which invests roughly $11 billion, did not provide details about returns on Friday and a spokesman declined to comment.

But gold was not the only bet that hurt the long-admired investor. A 23 percent drop in Micron Technology Inc, another one of Greenlight's top long positions, also hurt as did a 4 percent drop in Apple Inc's stock price, Greenlight's biggest bet.

On Friday, gold was at $1,094.91 an ounce, down 16 percent since the middle of January and off from a record peak of $1,900 hit four years ago.

Unlike mutual funds, hedge funds are not required to disclose their monthly returns and many managers do not disclose them, so any performance numbers, especially from big name managers, are scrutinized closely for industry trends.

The broader stock market Standard & Poor's 500 index gained 1.28 percent in July.

Meanwhile Daniel Loeb's $17.5 billion Third Point fared much better this month, posting a 1 percent gain its main fund, extending the year-to-date gains to 6.8 percent, an investor in the fund said.

Traditionally Einhorn and Loeb are among the first managers to inform clients of their returns every month. Other managers may take several days to finalize their numbers.

(Reporting by Svea Herbst-Bayliss; Editing by Lisa Shumaker)