The decision pushes Day into Saturday's elimination round of 16 and gives him critical rest after he hurt his back late in his opening match on Wednesday.
Casey had won the first two holes against Day and was still 2-up after four. But Day won the next two, and Casey was unable to continue and withdrew. Casey had lost his first match to Thongchai Jaidee on Wednesday and fought back late to tie Graeme McDowell on Thursday.
Casey was runner-up in the tournament in 2009 and 2010. He lost to Rory McIlroy in the 2015 quarterfinals.
Also Friday, Daniel Berger withdrew before his final round because of a left wrist injury suffered on the 18th hole the previous day.
The severity of Berger's injury wasn't immediately disclosed, but he had said that withdrawing was a possibility with the Masters just two weeks away.
Berger was injured during Thursday's match against Phil Mickelson when his club hit a rock wall on the downswing on his second shot on the last hole. He grabbed his wrist and conceded the hole to Mickelson, who won the match that had been all square.
The loss meant Berger's final scheduled match against Matthew Fitzpatrick was little more than a practice round, as neither player could advance to the weekend knockout rounds.
Information from The Associated Press was used in this report.