McDowell was at 12-under 132 with 48 players still on the course when the round was suspended.
The beginning of the round was delayed 2 hours and 15 minutes by high winds that blasted Pinx Golf Club in the morning.
Johan Edfors fired a 65 and Jeev Milkha Singh shot a 66 to share second place at 10-under 134, two shots behind McDowell.
Korea's Kim Hyung-Tae posted a 66 and led the home charge in the first European Tour event staged in his home country, joining Ireland's Paul McGinley (67) at 9-under 135.
Reigning British Open champion Padraig Harrington shot a 65 to share sixth place with Thomas Bjorn (66) and Anthony Kim (68) at 8-under 136. Terry Pilkadaris (66) is another shot further back at 7-under 137.
Chris DiMarco (68) led a group of six players knotted at 6 under, including four who were still on the course when the round was suspended.
McDowell kickstarted his bogey-free round Friday with birdies on his first two holes. He posted back-to-back birdies again from the fifth, and later made an eagle at the 543-yard 10th to move to 10 under.
The 28-year-old Ulsterman went on to birdie the 12th and 18th holes to match his second-best score on the European Tour. He has twice shot a 62, both coming in 2004.
'Shooting 64 today was very pleasant and I feel happy with what I am doing,' said McDowell. 'I feel I am putting well enough to really have a chance to win this tournament.'
Should he hold onto the 36-hole lead -- the nearest players still on the course were six shots back -- it would be only the third time in his European Tour career that McDowell led after two rounds.
McDowell owns two European Tour wins, the last coming at the 2004 Telecom Italia Open.
'I'm excited about the weekend now,' he said. 'I am striving for consistency this year and trying to give myself as many chances as possible to play at the business end of tournaments. I think this is the fifth time out of seven that I have given myself a chance at the weekend, so that's all I can ask for.'