* Futures down: Dow 153 pts, S&P 18 pts, Nasdaq 40 pts
By Tanya Agrawal
Aug 12 (Reuters) - U.S. stock index futures fell sharply for a second day in a row on Wednesday as China pushed the yuan lower again, exacerbating fears about a global economic slowdown.
* The yuan hit a four-year low with spot yuan dropping as low as 6.4510 per dollar, its weakest since August 2011, after the Chinese central bank set its daily midpoint reference at 6.3306, even weaker than Tuesday's devaluation.
* Commodity prices were also lower, with copper hitting a 6-year low. The 19-commodity Thomson Reuters/Core Commodity CRB index were at lows not seen since 2003.
* China's surprising move has led to speculation that the U.S. Federal Reserve will wait until December to raise rates.
* The probability of a rate hike next month faded to less than 50 percent, from a near 60 percent probability after last week's solid U.S. jobs data.
* The dollar index was slightly lower and failed to extend its gains against emerging market currencies across the board, on falling U.S. yields and Fed rate hike expectations.
* Automakers with a big exposure to China also fell again. Shares of General Motors Ford Motor and Fiat Chrysler were all down 1 to 4 percent premarket.
* Alibaba fell 5.1 percent to $73.37 after the retailer's revenue growth slowed to its lowest rate in more than three years.
* Apple fell 1.2 percent to $112.08 and was on course to dip to its lowest in more than six months.
* General Electric was down 1 percent at $25.45 after the company agreed to sell its U.S. healthcare finance unit to Capital One for about $9 billion.
* Data expected on Wednesday includes the U.S. Federal budget for July, which is likely to show a deficit of $132 billion, compared with a $51.8 billion surplus in June. The data is expected at 2 p.m. ET (1800 GMT).
Futures snapshot at 7:11 a.m. ET:
* S&P 500 e-minis were down 18 points, or 0.87 percent, with 320,454 contracts traded.
* Nasdaq 100 e-minis were down 39.5 points, or 0.88 percent, on volume of 51,549 contracts.
* Dow e-minis were down 153 points, or 0.88 percent, with 49,132 contracts changing hands. (Reporting by Tanya Agrawal in Bengaluru; Editing by Savio D'Souza)