The third round will conclude on Sunday morning ahead of the fourth round.
Wind gusts that approached 80km/h early Friday led to a four-hour delay to start the second round and left the tournament in catch-up mode the rest of the week.
Ghim made a big move on Saturday, birdieing his final four holes at TPC Summerlin while going seven under par through 16 holes. That left him at 15 under par overall.
Halfway co-leader Poston was two under through 13 holes to match Ghim at 15 under.
Kurt Kitayama and Gary Woodland each completed rounds of 65 on Saturday, leaving them a shot back at 14 under.
Also at 14 under are Harris English and Argentina's Alejandro Tosti, who are both four under through 14 holes in the third round.
Ghim logged three birdies and nine pars before he got hot, reeling off birdies at the par-5 13th hole, the par-3 14th hole, the par-4 15th hole and the par-5 16th hole before play was stopped.
Poston parred every hole he played in the third round except for one, when he eagled the par-5 ninth hole. He drove the green in two before rolling in a 3-foot putt.
Earlier on Saturday, Poston completed the wind-delayed second round by going birdie-eagle-birdie-par to leave him tied for the lead with Germany's Matti Schmid. He tied for third in the event last year.
"I like this golf course," Poston said ahead of the start of the third round. "It sort of the fits my eye. The greens are bentgrass, what I grew up on. A lot of familiar stuff about this week that I enjoy.
"It's a tournament I love coming back and playing. Obviously would like to finish it off. I had a good chance last year I felt like, just never quite finished my rounds off last year. Hopefully can change that this year."
Australians Aaron Baddeley (four over) and Cam Davis (11 over) were also well outside the cut line after finishing their second rounds on Friday.
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