Paul Casey withdrew from the Dell Match Play early in his match Friday with Jason Day because of illness.
The decision pushes Day into Saturday's elimination round of 16 and gives him critical rest after hurting 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.