Russell Knox started the WGC - Dell Technologies Match Play in a rich vein of form as he led two up against Bill Haas after 12 holes at Austin Country Club.
Knox - playing in just his second European Tour event of the season - did not drop a shot on his front nine as he piled the pressure on his American opponent in the early exchanges of his match.
The Scotsman opened up in the best possible way after a par on the first to go one up before extending his lead on the third with another par after Haas carded a bogey.
Haas halved the deficit with a wonderful chip to within a foot to pick up his first birdie on the sixth but Knox restored his two up advantage with his first gain of the round after sinking a 15-foot putt on the ninth.
Knox made it successive birdies on the next when his approach landed six feet from the pin to go three up and put him in pole position for victory in his Group 11 match, despite a bogey on the 12th to down a shot.
K.T. Kim was victorious over 2016 Masters Tournament champion Danny Willett following a 3 and 2 win in the other Group 11 encounter.
Kevin Na stormed into a five up advantage over Matthew Fitzpatrick through 11 holes to take complete control of their Group 6 clash.
A par on the first was enough for the American to develop his lead before he extended it to two up at the third when he picked up his first birdie of the round.
A fantastic run of eagle-birdie-birdie on the sixth, seventh and eighth transformed Na's lead to five up before pars at the next two, with Fitzpatrick with it all to do to stay in the match..
Chris Wood was all square with Justin Thomas in a see-saw battle after 11 holes, which saw the American take the initiative when his tee-shot on the fourth stuck five feet from the pin to go one up.
The two players exchanged birdies on the fifth and sixth as Thomas maintained his advantage before the Englishman levelled the scores with a par on the eighth following his nine-foot putt which the World Number Seven could not match.
In Group 3, Lee Westwood was one down to Marc Leishman through seven holes after the Australian picked up a birdie on the opening hole before par handed him a two up lead following the Englishman's bogey on the third.
Westwood replied with his first birdie of the match on the fifth to reduce the deficit with 11 holes still to play.
Defending champion Jason Day will play no more part in the tournament after he withdrew to be with his mother, who is due to undergo lung cancer surgery.
In Group 14, Daniel Berger led three up against J.B Holmes through seven holes, while Phil Mickelson was four up over Si Woo Kim after eight.
