Day (3-0) was the first player to advance to the round of 16, a surprising result considering his status for the WGC-Dell Match Play was uncertain after he limped off the course Wednesday because of back pain.
“Look, I’ll take it,” he said afterward. “It’s definitely a good thing for me to be able to get some more rest and try and rest my back and get some good therapy going on my back.”
Day breezed through Thongchai Jaidee, 5 and 3, in the second round and was all square through six holes with Casey when the Englishman conceded the match because of a stomach virus.
The No. 2 overall seed, Day will play the winner of the Charl Schwartzel-Brandt Snedeker match on Saturday morning.
Last year, Casey also fell ill during his quarterfinal match against Rory McIlroy, ultimately losing in 22 holes.