Star pair Rory McIlroy and Jordan Spieth were shock losers as the WGC Dell Technologies Match Play got underway in Austin on Wednesday.
McIlroy and Spieth lost early in the round-robin group stages of the event, going down to Dane Soren Kjeldsen (2&1) and Japan's Hideto Tanihara (4&2) respectively.
Kjeldsen made an early gain on former world number one McIlroy with a birdie at the first in Group Two and was two up on the Northern Irishman at the 10th.
McIlroy turned things around with the help of a mammoth drive at 12, delivering three birdies on the spin to edge in front of the Dane. However, Kjeldsen hit back with gains on the 14th and 16th holes and landed within a foot of the flag from his drive at the 17th to close out the match.
Two-time major champion Spieth - playing in Group Five - never led in the match as Tanihara wrapped up the contest with two holes to spare.
The most shocking story of the day came from Jason Day, who withdrew due to family circumstances after losing three of his first six holes to Pat Perez.
Many initially believed that the Australian's back had acted up again, but unfortunately it was a much more serious issue. Day said in a brief news conference that his mother is undergoing surgery to remove a tumour in her lung on Friday.
Each player in Day's group will earn a point for his departure from the competition.
Group One, which was pegged as the 'Group of Death', lived up to its name. World number one Dustin Johnson obliterated Webb Simpson 5&3, while Martin Kaymer outduelled Jimmy Walker 3&2 in a matchup between PGA Championship winners.
Johnson and Kaymer are two contenders in this tournament, but only one will make it out of the group.
In other notable matches, Justin Thomas defeated Englishman Chris Wood 2&1, Phil Mickelson defeated So Woo Kim 5&3, Hideki Matsuyama and Jim Furyk halved and Brooks Koepka snapped out of his shockingly bad form to beat Kevin Kisner 6&5.
Pool play will continue on Thursday and Friday. The 16 winners from each group will then compete in single elimination over the weekend.