McDowell made three birdies in a five-hole stretch to take a 4-up lead at the turn, and then outlasted his Irish friend, 3 and 2, on Saturday to reach the quarterfinals of this World Golf Championship for the first time.
Lowry became the first No. 64 seed in the 15-year history of the event to reach the third round, after an 18-hole win over No. 1 Rory McIlroy and a thrashing of Carl Pettersson. Lowry just wasn't sharp enough to handle McDowell.
"Very difficult to get the poker face on and really knuckle down and play Shane, being a good friend like he is," said McDowell, who knocked out both players from Ireland this week after beating Padraig Harrington in the first round.
Next up for McDowell on Saturday afternoon was Jason Day, who eliminated Masters champion Bubba Watson and left this tournament without a top-10 seed going into the quarterfinals for the first time ever.
The best golf belonged to Steve Stricker, who celebrated his 46th birthday with a match that made him feel 47. Stricker made eight birdies and needed every one of them to beat Scott Piercy, who shot 65 and lost. Stricker was 1 down with two holes left when he rolled in a 5-foot birdie putt on No. 8 and won the match with a 30-foot birdie putt on the ninth hole. Piercy played his last 30 holes in 14-under par, without a single bogey, and he was out of the tournament.
"I don't think either one of us made bogey," Stricker said, watching Piercy walk alone toward the clubhouse at Dove Mountain. "That's match play, I guess. Neither one of us deserved to lose."
Stricker faced his toughest match that afternoon — Ian Poulter, the guy no one wants to play and everyone wants to beat.
Poulter, quickly developing a reputation as the best in match play on the strength of his Ryder Cup performances, finally pulled away from Tim Clark when the South African made a pair of mistakes at the turn. Poulter went to 3 up and wound up with a 5-and-3 victory.
In other third-round matches Saturday morning:
— Robert Garrigus went another match without reaching the 18th hole, and he joined defending champion Hunter Mahan as the only players who have not trailed this week. Garrigus birdied the second hole and went on to a 3-and-1 win over Fredrik Jacobson of Sweden.
He next faced Matt Kuchar, who got ahead early of big-hitting Nicolas Colsaerts and won, 4 and 3.
— Mahan has played only 43 holes in three rounds, and he extended his streak to 133 holes without trailing dating to the opening round last year. Mahan got a small measure of revenge against Martin Kaymer, the last player to beat Mahan on Dove Mountain.
Mahan had a quarterfinal match against U.S. Open champion Webb Simpson, who needed all 18 holes for a 2-up win over Gonzalo Fernandez-Castano of Spain.
Five Americans reached the quarterfinals, the most in 10 years.
Poulter was the highest seed remaining at No. 11.
McDowell's win was good news for at least one person in Ireland. By keeping Lowry from reaching the semifinals, he assured Harrington would drop no further than No. 50 in the world and qualify for the next WGC event in two weeks at Doral.
Alex Noren, whom McDowell beat in the second round, will move to No. 49 and get into Doral.