Phil Mickelson withdrew from the Memorial Tournament in Dublin, Ohio, on Thursday after one round, citing mental fatigue.
"I think mentally I'm a little bit fatigued," Mickelson told reporters, after shooting an opening-round 79. "I feel like it's the responsibility of a player to see through your commitment and finish the tournament and so forth, and I'm kind of overruling that just a touch because I'm trying to think big picture on what's the best way for me to get ready for the U.S. Open."
The U.S. Open is in two weeks.
Mickelson had played three consecutive tournaments earlier in May, and, after that, he took his wife, Amy, to Europe to celebrate her 40th birthday.
The 79 is his worst score in his 12 appearances at Muirfield Village.
"The course here is in such great shape. It's a beautiful way to get ready for the U.S. Open," he said. "But I'm hitting it so poorly that ... I have to look at what's best for me to play in the U.S. Open, and I'm going to take the next few days to kind of rest up."
The last time Mickelson withdrew from a tournament was also at the Memorial, in 2007, when he injured his wrist.