Rory McIlroy remained unbeaten, Tiger Woods suffered a critical loss and defending champion Bubba Watson and two-time winner Jason Day were eliminated at the WGC-Match Play Championship.
The 64-player showdown featuring the world ranking leaders at Austin (Texas) Country Club starts with three days of group play to determine 16 berths in weekend knockout matches to decide a champion.
Fourth seed McIlroy defeated South Africa's 47th-seeded Justin Harding 3&2, eagles at the par-5 12th and 16th and a birdie at the par-4 15th tipping a squared match his way to give him a 2-0 record and group lead.
"One of the big things over the last couple of days is I've been happy with how I've responded. I played the shot when I needed to -- I holed the putt when I needed to.
"When I sense a little bit of blood I'm taking my opportunities. Whenever I have to step up and hit a shot on top of a guy I've been able to do that really well."
McIlroy, who has never trailed in two days, needs only a draw with English 32nd seed Matthew Fitzpatrick on Friday to advance.
Brandt Snedeker never trailed in beating 14-time major champion Woods 2&1 to share the lead in an all-American group with Patrick Cantlay, who beat Aaron Wise 4&2.
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For any chance to reach the weekend in his first Match Play start since 2013, three-time winner Woods must beat Cantlay on Friday and have Wise defeat or draw Snedeker.
"We both made a couple of mistakes out there but he made a few less mistakes than I did," Woods said.
"All I can do is hopefully get a point tomorrow and see if that's good enough."
Woods made an incredible third shot at the 10th hole on his knees, knocking the ball from underneath a huge bush some 32 feet to just four feet from the cup and halved the hole.
"My own kids were probably rooting for him," Snedeker admitted.
Snedeker sank a nine-foot birdie putt at the par-5 17th for a 2-up lead and halved 17 to seal the victory.
"It was a lot of fun," Snedeker said.
"Neither one of us made a lot of putts. It came down to me making a good putt on 16 and that gave me the little bump I needed to win."
- Rose rallies to halve -
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England's second-seeded Justin Rose was 3-down with four holes remaining but charged back with long birdie putts to halve with compatriot Eddie Pepperell on a day when 17 of 32 matches went to the 18th hole.
Rose birdied to win the 15th from 13 feet, sank a 23-footer at the par-3 17th to stay in the match, then closed the duel on a tense 10-footer at 18 to finish level.
"My goal was to birdie out," Rose said of the final four holes. "You set your intention and forget the rest and that's what happened."
Rose must beat US 22nd seed Gary Woodland, a 1-up winner over Argentina's Emiliano Grillo, on Friday to advance.
Watson was eliminated with a 2&1 loss to fellow American Billy Horschel while 12th seed Day, a two-time champion, was ousted with his second defeat, falling 4&3 to Sweden's Henrik Stenson.
Stenson plays Jim Furyk, who eliminated five-time major winner Phil Mickelson 1-up, on Friday to decide a spot in the last 16.
- Grace beats No. 1 DJ -
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South Africa's 40th-seeded Branden Grace beat world number one Dustin Johnson 1-up. A 38-foot birdie putt at 17 by Grace proved the difference after Johnson missed a 17-foot birdie putt at 18.
"It's always nice to beat the number one player in the world," said Grace, who can do no worse than a playoff with Johnson -- who must beat Japan's Hideki Matsuyama and have Grace lose to American Chez Reavie to force it.
China's Li Haotong edged US third seed Brooks Koepka 1-up to eliminate the reigning US Open and PGA champion from title contention.
Aussie Marc Leishman beat Scotsman Russell Knox 2-up for the group lead after Thailand's Kiradech Aphibarnrat beat US sixth seed Bryson DeChambeau 2&1.
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