Masters favourites Rory McIlroy and Jordan Spieth suffered opening-day defeats at the WGC-Dell Technologies Match Play as Jason Day withdrew from the event amid emotional scenes in Austin.
McIlroy suffered a surprise 2&1 defeat to Soren Kjeldsen, who was six under for his round in claiming his first ever win in the tournament, while Spieth looked out of sorts in a 4&2 loss to Hideto Tanihara.
Defending champion Day then conceded his match to Pat Perez after only six holes, and he headed straight to the media centre to explain he was in no fit state to play golf due to his mother being diagnosed with lung cancer.
World No 1 Dustin Johnson cruised to a 5&3 victory over Webb Simpson, while English trio Paul Casey, Tommy Fleetwood, and Tyrrell Hatton and Scotland's Russell Knox all got off to winning starts.
McIlroy was out early and hit back from two down after 10 to win three consecutive holes, only for Kjeldsen to reel off four birdies in a row in an outstanding display of ball-striking and assured putting, finishing off the match with a sublime tee-shot to three feet at the par-three 17th.
Spieth made only one birdie in his clash against Tanihara, who won the 12th and 15th with pars to go three up before being conceded the match at the next with the American still in the rough after three shots.
Ryder Cup sensation Thomas Pieters was a handsome 3&2 winner over Scott Piercy, while in-form fourth seed Hideki Matsuyama was held to a halved match by veteran Jim Furyk.
Last year's runner-up Louis Oosthuizen thrashed Ross Fisher 4&3, and Fleetwood won the final hole of a tense tussle with two-time major champion Zach Johnson with a classy birdie after pitching to three feet with his second.
Johnson's bid for a third straight victory this year got off to an encouraging start as he won the first three holes against Simpson and effectively put the match beyond reach when he went five up at the turn.
Simpson did well to extend the contest with winning birdies at the 11th and 13th, but Johnson closed out a comfortable win two holes later and takes on Martin Kaymer on Friday in a match which should decide the top group after the German saw off Jimmy Walker 3&2.
Danny Willett's struggles ahead of his defence of the Masters continued as he slumped to a 4&2 defeat to KT Kim, while Knox was always in control of his 3&2 win over Bill Haas.
Matt Fitzpatrick could not reproduce the form he showed as he contended for the Arnold Palmer Invitational title last week, carding five bogeys in a disappointing 5&4 loss to Kevin Na.
There were two one-sided contests in group 14, with Phil Mickelson easing to a 5&3 win over Si Woo Kim and Daniel Berger storming to the biggest victory of the opening day - a 7&5 drubbing over JB Holmes.
Marc Leishman was presented with an Arnold Palmer-inspired red cardigan for his win at Bay Hill last week, and the Australian admitted he had it in his bag during a 3&2 win over an erratic Lee Westwood.
Tenth seed Tyrrell Hatton was one down at the turn against Charles Howell III, but the young English talent took full advantage of some ragged play from his opponent on the back nine - winning the 13th and 14th with pars before capping a 2&1 win at the penultimate hole.
Rafa Cabrera Bello and Jon Rahm flew the flag for Spain as they beat Jeunghun Wang and Kevin Chappell respectively, while Sergio Garcia snatched a draw when Shane Lowry hit a wayward drive at the last and compounded the error when he three-putted from long range.
But events on the course were put into perspective when Day struggled to hold back the tears following his concession to Perez, revealing he had been finding it difficult to focus on the game since learning of his mother's diagnosis at the start of the year.
Day did not give any indication of whether he would be at Augusta National in a fortnight, although he did announce that he was free of injury after speculation he had suffered another health setback ahead of the first major of 2017.