McDowell started the day leading by two shots clear, but a double bogey on the 18th gave him a 6-under 66 and a total of 18-under 198, tied with Singh and four shots ahead of the rest of the field.
Singh had nine birdies against a single bogey.
Paul McGinley of Ireland shot a 5-under 67 and was in third place. Johan Edfors of Sweden was another shot back in fourth after a 69.
British Open champion Padraig Harrington carded a 4-under 68, to lag seven shots behind the lead.
Tournament and local favorite K.J. Choi shot a 3-under 69 and was 17th at Pinx Golf Club, site of the $2.9 million event, jointly sanctioned by the European, Asian and Korean tours.
I still havent adapted well to the speed of the greens and I made many mistakes today, said Choi, ranked No. 5 in the world. If the weather is fine tomorrow, I will do my best to reduce the gap at the top.