Jason Day, who has been battling a back injury, clinched a place in this weekend's knockout phase of the WGC-Dell Match Play when Englishman Paul Casey withdrew after six holes due to illness, with the score all square.

The win gave Day, who walked off the Austin Country Club course on Wednesday clutching his back due to spasms ignited by a tee shot late in his opening match against Northern Irishman Graeme McDowell, a perfect 3-0-0 record.

Casey had won the first two holes of their duel on Friday and was 2-up until losing the fifth and six holes and then withdrawing.

The Englishman, runner-up in the tournament in both 2009 and 2010, had lost his first match to Thongchai Jaidee of Thailand on Wednesday and fought back late to tie McDowell on Thursday.

World number two Day, the PGA Championship winner, will play American Brandt Snedeker in Saturday morning's round of 16.
Source: AFP