Casey defeated J.B. Holmes in a playoff here last year to pick up his first PGA Tour win.
HUMBLE, Texas (AP) -- Vijay Singh withdrew from the Houston Open on Thursday with a back injury, the second time in three weeks that it's flared up.
Singh played nine holes in the first round, shooting a 43 with two double bogeys, before walking off the course. He also withdrew from the Transitions Championship two weeks ago with a back injury.
"I think it's the recurrence," Singh said. "It's not as bad as what it was in Tampa, but I think if I kept going, it was going to go that way."
Singh tied for fourth at the Honda Classic and tied for 11th at Doral before the Transitions event. He said he'll take the next four days off and "see what happens."
"It affects me when I'm hitting balls," he said. "It's not my normal golf swing."
Earlier Thursday, defending Houston Open champion Paul Casey withdrew because of a left shoulder injury. The 32-year-old Englishman is hoping the latest injury is "just a one-week thing" and won't keep him out of next week's Masters.