Summerhays has finished in the top 100 in the FedExCup for each of the past five seasons, and a year ago finished T8 at the U.S. Open and third at the PGA Championship.
Now, entering Sunday's final round, Summerhays, who never has won on the PGA Tour, will try to protect a three-shot lead after shooting a moving-day 4-under 68.
Victorian youngster Brett Coletta, playing just his second US PGA Tour event, shot 72 to remain at three under (tied 27th) and is five shots ahead of countryman Cameron Smith (73) at two over.
That is ten shots behind leader Jason Dufner who incredibly shot back to back rounds of seven under par 65 to lead the rest of the high quality field by five shots.
Dufner's scorecard included five birdies and an eagle.
"I felt like I scored the way I played today", Spieth said.
Dufner, whose final moment of indignity came on No. 18, where he lost his tee shot in the creek to the left of the fairway and bogeyed to close out a 77, is 9-under along with Bubba Watson and Justin Thomas.
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"If I can drive it like this at the US Open, a major where it's so crucial to drive it straight, it should be a good week", said Leishman. "Even if it doesn't shake out how I want to [Sunday], I think I'll be like, 'Oh, where's the next hole I need to go play?"
CHASING: Dufner was so smooth through two rounds and appeared unstoppable.
Scott was the best of the Australians on day two but it is Victoria's Marc Leishman who is best placed overall after adding his second consecutive 70 to be 4-under and remain T13. The world's No. 1 player three-putted the par-3 16th after hitting into the water and made triple bogey.
The 33-year-old hit 13 of 14 fairways and said sustaining his career-best form with the driver would be key to any hope of catching Dufner. Fowler (66) was another shot behind. Coming off a 78 - his first round without a birdie in almost four years - Johnson couldn't get anything going and shot 74 to miss the cut by five shots. When asked about it at Kapalua to start the year, Dufner said: "I've been putting bad for 17 years".
Daniel Summerhays made bogey on his last hole and shot 69 to finish five shots behind Dufner.
His second round play was enough to move him 15 places up the leaderboard and at 3-under he is just one shot behind the veteran Leishman.
Sun-splashed skies and calm winds yielded low scores in the first round of the Memorial Tournament at Muirfield Village Golf Club on Thursday.
Taking the week off, Jutanugarn replaced Lydia Ko atop the ranking when So Yeon Ryu missed the cut. "I'll probably hit a few putts with the hands shaking and everything like that".