* Oil falls 6 percent, Exxon tumbles in pre-market
* Airline shares rally on drop in crude prices
* Indexes could notch sixth straight weekly advance
* Dow up 20 pts, S&P down 1 pt, Nasdaq up 12.75 pts (Updates prices)
By Ryan Vlastelica
NEW YORK, Nov 28 (Reuters) - U.S. stock index futures pointed to a flat open on Friday, as the market's recent upward bias looked to continue in a holiday-shortened session, though energy shares saw heavy pressure as crude oil tumbled to a four-year low.
Wall Street was closed for the U.S. Thanksgiving holiday on Thursday and will shut three hours earlier on Friday. Trading volatility could spike since many market participants were extending the holiday.
U.S. crude futures slumped 6 percent to $69.26 per barrel. It was the biggest one-day drop for crude since September 2011, and the decline brought prices to their lowest since 2010. On Thursday, OPEC decided not to cut output, which could leave oil markets heavily oversupplied.
Energy shares have been under pressure throughout 2014 as crude prices have slid more than 30 percent from a recent closing high. The S&P energy sector is the only S&P 500 industry group to be lower for the year, down 4.3 percent.
The Energy Select Sector SPDR exchange-traded fund fell 4 percent to $81.88 in pre-market trading. Exxon Mobil Corp lost 3.5 percent to $91.20 while Chevron Corp fell 3.4 percent to $111.21; both stocks are Dow components. Halliburton Co lost 5.3 percent to $44.81.
The weakness in oil could be a boon for the holiday shopping season, with lower gas prices possibly sparking increased consumer spending.
The decline in oil boosted airline shares ahead of the market open. American Airlines Group Inc rose 5.4 percent to $47.40 before the bell, while JetBlue Airways Corp was up 5.4 percent to $14.33. Delta Air Lines Inc jumped 4.9 percent to $46.40.
Despite the energy weakness, major indexes could eke out a sixth straight weekly advance. For the week so far, the Dow is up 0.1 percent and the S&P is up 0.5 percent. The Nasdaq, which counts few energy names among its major components, is up 1.6 percent this week.
Equities have been strong of late, with the Dow and S&P 500 both closing at records on Wednesday. Recent gains have been fueled by actions from central banks around the world, as well as strong economic data and corporate results.
Futures snapshot at 8:49:
* S&P 500 e-minis were down 1 points, or 0.05 percent, with 174,243 contracts changing hands.
* Nasdaq 100 e-minis were up 12.75 points, or 0.3 percent, in volume of 30,142 contracts.
* Dow e-minis were up 20 points, or 0.11 percent, with 32,705 contracts changing hands. (Editing by Paul Simao and Nick Zieminski)