By Helen Ross, PGATOUR.COM DUBLIN, Ohio -- Citing mental fatigue, Phil Mickelson withdrew from the Memorial Tournament after shooting a 79 on Thursday. The round of 7 over was Mickelson's highest in the 43 he's played at Muirfield Village. The Memorial Tournament was Mickelson's fourth start in the last five weeks, and he spent the other in France and Italy celebrating his wife Amy's 40th birthday. "I came back and had a Tuesday outing in Long Island, the pro am and I think mentally I'm a little bit fatigued," Mickelson said. "So the course here is in such great shape. It's a beautiful way to get ready for the U.S. Open. 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. Mickelson, who shot a 42 on the back nine that included five bogeys, one double bogey, a birdie and two pars, said he would probably seek out his instructor Butch Harmon in the 10 days before the U.S. Open at the Olympic Club. Mickelson has five runner-up finishes in the season’s second major. "Certainly I'm disappointed with how I played today, but I think ... I've got to be more big-picture oriented and think about the Open and what's best to get my best golf out there, and I need the next few days to rest up a bit," Mickelson said. Amy Mickelson's 40th birthday was actually Thursday so her husband planned to head home immediately to see her. Mickelson, who tied for seventh in his last start at the HP Byron Nelson Championship, said the couple had a "great time" on their European vacation "But I think I probably just went a little bit overboard last month, and it has nothing to do with playing poorly and so forth, but I do think I need to get rested to play my best for the Open," Mickelson said. Mickelson, whose brother Tim is trying to qualify for the U.S. Open in Columbus on Monday, said he might try to head up to San Francisco for a scouting mission on the Olympic Club. “I haven't had a chance to get out there yet,” Mickelson said. “I know it's a course that's very strategic that you have a lot of options off the tee and you have to have a solid game plan heading in, and I haven't had as much time there as I'd like.”