Day notched his first PGA Tour victory at TPC Four Seasons back in 2010, and his record at this event included a fifth-place finish in 2011 and a T-9 result in 2012. At No. 8 in the OWGR, Day was the second-highest ranked player in the 156-man field behind only world No. 2 Jordan Spieth.
While no reason was originally offered when the Tour announced Day's withdrawal, multiple onlookers noted that the Aussie played five holes of his pro-am round before walking off the course. Bill Nichols of the Dallas Morning-News then tweeted that Day withdrew because of "severe dizziness":
.Jason Day's WD from AT&T Byron Nelson caused by "severe dizziness" during Wednesday pro-am— Bill Nichols (@BillNicholsDMN) May 27, 2015
A winner earlier this season in San Diego, Day has not played since shooting an 81 in the second round of The Players Championship. He has a home in Columbus, Ohio and is still listed among the early commitments for the Memorial Tournament next week.
Day was replaced in the field in Dallas by Tim Petrovic.