* Apple up after Topsy buy, UBS upgrade
* Cyber Monday sales appear strong
* Yum Brands November China sales flat
* Futures down: Dow 68 pts, S&P 5.2 pts, Nasdaq 4.75 pts
NEW YORK, Dec 3 (Reuters) - U.S. stocks were poised for a lower open on Tuesday, indicating the S&P 500 may fall for a third consecutive session, on concerns the Federal Reserve may begin to reduce its stimulus earlier than some had anticipated.
The benchmark S&P index fell on Monday as stronger-than-expected data on manufacturing and construction spending prompted speculation the Fed may lean towards scaling back its stimulus of $85 billion in monthly bond purchases.
While virtually all market participants accept the central bank will begin to trim the stimulus at some point, the timing remains in question, with many analysts expecting the announcement in March.
"It's not a question of if, it's a question of when and you always have the question of how much of it is real and how much of it is emotional in nature?" said Gordon Charlop, a managing director at Rosenblatt Securities in New York.
After an eight-week run of gains for the S&P 500 that saw the benchmark index hit a series of record highs, market participants may also be engaged in some minor profit-taking.
"The near term trend seems to be to the downside, but that being said I am not going to sit here and say we are sensing the beginning of some sort of three- to five-percent correction or something more significant than that," Charlop said.
The Fed has said it would begin to slow the program when certain economic measures meet its targets. The economic calendar is packed this week with data that may provide some insight, culminating with Friday's November payrolls report.
S&P 500 futures fell 5.2 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 68 points and Nasdaq 100 futures lost 4.75 points.
After a disappointing to start to the holiday shopping season, U.S. online sales are expected to hit $2 billion on "Cyber Monday," the highest since the data firm comScore began tracking such information. Amazon.com Inc shares slipped 0.8 percent to $389.36 in light premarket trade.
Apple Inc has acquired social media search and analytics startup Topsy, an unusual purchase for a hardware-focused company that has made few forays into social networking. UBS upgraded the iPhone maker to a "buy" rating. Apple shares were up 1 percent to $556.61 in premarket trade.
Yum Brands Inc lost 2.3 percent to $75.90 in premarket after it said November sales at established KFC restaurants in China, its top market, failed to grow despite a successful half-priced chicken promotion, and it forecast a return to earnings per share growth in 2014.
OncoMed Pharmaceuticals Inc surged 80.7 percent to $25.30 before the opening bell after the company said Celgene Corp would develop and market six of its anti-cancer stem cell experimental drugs for an upfront payment of $155 million.