Sunday, he had six birdies and four bogeys to shoot a 2-under 69, finishing the week with four straight rounds in the 60s.
The world number 14 began the day with a two-stroke lead and, early on, looked set for a comfortable stroll to victory at Quail Hollow as he birdied the second. In fact, for the better part of the last two decades the top ranking was something of an afterthought, the byproduct of Tiger Woods' stranglehold on the top spot.
"To be clutch like that, it's a lot of heart, " Day said. "And you never know, hopefully I can come into a week like this and really hit a lot of greens and give myself opportunities". I had no confidence in my ability to hit proper tee shots.
He won for the 12th time in his career.
After squandering a three-shot lead on the back nine, Day made amends with back-to-back- birdies late on Sunday to clinch his 12th PGA Tour victory in style.
Langer had to rally from as many as four shots behind in the final round at the The Woodlands Country Club.
Uihlein, who started the round at two over, carded five successive birdies from the fifth before hitting an eagle at the 10th to go seven under in six holes, adding two more gains at the 14th and 15th to cap off a stunning display.
He was right of the fairway among pine trees blocking his path over the water to the green. Rickie Fowler and Justin Thomas were one shot behind him.More news: Anthony Martial sends message to Arsenal boss Arsene Wenger
"I didn't putt well again", Woods said.
Day was unlucky not to make a hole-in-one but the stick also saved his ball from rolling off the back of the green.
Bernhard Langer won at 11-under, one ahead of Paul Goydos, Jeff Maggert and Bart Bryant. "My next step is to try to win a major this year". It was Langer's fourth win at the Insperity Invitational.
This was his 38th worldwide start since that win, and there were all manner of issues that cropped up. He had lost in playoffs his previous two tournaments. And when Day finally picked up his first birdie by blasting a 9-iron out of the rough with the wind at his back from 195 yards to 18 inches on the ninth hole, he was on his way.
Rookie Aaron Wise was tied with Day as he teed it up on No. 16. However, she finished 90 minutes before the tournament ended because groupings were not changed from the opening round. McIlroy three-putted the last hole and still shot 66.
Park was the LPGA Tour rookie of the year and shared the LPGA player of the year award with So Yeon Ryu last year.
Day, who is from Australia but lives in Columbus, Ohio, said watching highlights of LeBron James' game-winning exploits for the Cleveland Cavaliers on Sunday morning did not hurt his plight as he struggled through some tough moments on the course. McIlroy has multiple top 10s on this course, but his final position on the leaderboard has gotten progressively worse every year since 2014. And it worked out fine, just like the rest of his round. The 23-year-old has not made a halfway cut in eight events this season.