Meeting each other for the first time in head-to-head competition, McIlroy three-putted the first and then fell two down after hitting into the lake on the long third.
It was a clash between the players ranked fifth and sixth in the Official World Golf Ranking. McDowell is the higher of the pair and he was clearly in a hurry, playing his approach to the opening hole while a greenside bunker was still being raked from the game ahead.
McIlroy, meanwhile, was clearly in no mood to be generous to his close friend.
After missing his six foot par putt on the first he asked McDowell to hole from half the distance for the win and after they were both bunkered at the short second McIlroy did not concede an even shorter putt as both got up and down.
Luke Donald and Martin Kaymer, both seeking to take the World Number One spot off Lee Westwood, were off to good starts too.
Kaymer came back from losing the first to be three up on Dane Søren Kjeldsen after six, while Donald took the third and driveable fourth to lead Swede Johan Edfors by two - the second of them with an eagle two.
Top seed Westwood was in the last game out and was involved in another all-Ryder Cup clash with Ian Poulter.
He was in trouble at the first after driving into sand and failing to make the green, but got up and down for a half after Poulter missed from 14 feet.
Defending champion Ross Fisher, meanwhile, was one down to Masters Tournament champion Charl Schwartzel after driving into the bushes on the long fifth.
Spain's Alvaro Quiros did the same against Korean Noh Seung-yul, but was one up after seven, while Venezuelan Jhonattan Vegas stood one up on Belgian Nicolas Colsaerts after four and Italian Francesco Molinari two up on Australian Aaron Baddeley after three.
McIlroy won the fifth to cut the gap, but McDowell was two up again thanks to a birdie two on the short next.
The last 16 involved eight of last year's Ryder Cup side, the Masters champion and five of the top six in the world.
Westwood nosed in front of Poulter with a birdie on the fourth, only to lose the next to a birdie four, while Donald was pulled back to all square when he ran up a triple-bogey seven at the seventh and then could not match Edfors' birdie at the par five eighth.
Fisher, who came from four down to beat American Ryan Moore on Friday, turned two down to Schwartzel and Kaymer was three up on Kjeldsen with eight to play.
McIlroy, on in two at the long eighth, three-putted again and fell three down, but he pulled out an invaluable 18 footer on the next to avoid turning four behind.
McDowell halved it in birdies from five feet and was out in a superb five under 32, while Westwood fell behind to Poulter when he bogeyed the short sixth and Donald was suddenly trailing as well when he bogeyed the tenth and Edfors birdied the next.
Kaymer, with his brother Philip as caddie this week after a parting of the ways with Scot Craig Connelly, looked set to be the first into the quarter-finals when a ten footer took him four up on Kjeldsen with five to play.
McIlroy missed a chance to win the short tenth, but worse was to come when he failed to hole from seven feet after McDowell had made a 12 footer on the next. That made it four up with seven to play.
Westwood birdied the eighth and ninth, but Poulter matched him both times with good putts and so started for home one ahead, while Donald remained one down with five to go.
Kaymer was the first into the quarters, finishing off Kjeldsen 3 and 2, but he had to wait to discover his afternoon opponent as Quiros and Noh were level coming down the final hole.
McIlroy threw his putter after losing the long 11th, despite being closer in three, and kicked his ball away when he missed a chance to take the next, but he did hole from the fringe of the 13th and cut the deficit to three.
Fisher almost pulled another remarkable recovery out of the bag. He birdied the short 17th to be only one down, then eagled the par five last - but Schwartzel matched it after hitting a wood to six feet and holing.
Noh made a mess of the same hole and so it was Quiros facing Kaymer, while McIlroy's birdies at the 13th and 14th and a 12 foot par putt on the next kept his hopes alive.
He was two down with three to play, however, and Westwood went two down when he bogeyed the 12th.
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