Martin Kaymer, who could replace Tiger Woods atop the world rankings with a high finish at Valderrama, carded a 1-over 72.
The 25-year-old Kaymer covered his first 10 narrow and tree-lined holes in 1 under then had a run of three bogeys.
His missed from 18 inches on his way to three-putting the 11th green, found a bunker off the tee at the next, and drove into trees at the 13th.
A birdie from two feet at the 16th repaired some damage for Kaymer, who won his first major at this year's PGA Championship.
The German can take over No. 1 from Woods by winning or finishing tied with no more than one other player for second.
But he said his main priority was the Race to Dubai, which he leads from second-placed McDowell by €995,561.
McDowell finished with a measured four-foot putt on the 18th green for his third birdie of the round.
Another Ryder Cup player, Ross Fisher, stood on the 18th at 2 under but found trouble in the cork trees lining the fairway. After a double bogey six he shot an even-par 71.
In second to McDowell in the clubhouse was his fellow Northern Irishman Gareth Maybin at 2-under 69.
Ryder Cup player Franceso Molinari shot a 70.
Meanwhile, the European Tour’s youngest winner, Matteo Manassero, confirmed his berth in next week’s HSBC Champions in Shanghai.
The 17-year-old Italian had initial trouble obtaining a visa at short notice, but his management company IMG said that hurdle has been overcome.