By Mark Lamport-Stokes

OAKMONT, Pennsylvania (Reuters) - South Africa's Louis Oosthuizen "found something" in his swing out on the course before plundering five birdies on his back nine to soar into contention at the weather-disrupted U.S. Open on Saturday.

Blessed with one of the most envied swings in the game, Oosthuizen rebounded from an opening five-over-par 75 to fire a five-under 65 on a challenging layout that was firming up at a sun-splashed Oakmont Country Club.

The 33-year-old had three holes to complete in the delayed second round when he returned to the course on Saturday morning and he finished par-par-birdie to cap an inward half of four-under 31 and match the best score of the week.

"I didn't have a great first round, I felt as uncomfortable over the golf ball as I have in a while," 2010 British Open champion Oosthuizen told reporters after sinking a 12-footer on his final hole, the par-four ninth.

"Couldn't really find anything on the range, as well, before the second round. But out on the golf course, I found something in my setup that felt comfortable, and I started swinging a bit.

"I started getting my little cut back on my driver and it went from there. Built momentum by hitting better shots and rolling a few nice putts. I hit a few shots really close."

Oosthuizen said he had also benefited from his memories of last year's U.S. Open at Chambers Bay where he tied for second after battling back from an opening 77 with scores of 66, 66 and 67.

"What I learned from last year was to never really give up in a U.S. Open," said the world number 14. "Just grind on. If something happens, then you can get yourself right back into it.

"I was just grinding the whole day. It was a great round coming back after not a great first round. I could have easily just gone out there and shot another 74 or something and be out of the tournament."

Oosthuizen birdied five of his last seven holes to post an even-par total of 140, just four strokes off the early lead.

"Just puts you back in the tournament really," he said. "There's a lot of golf left. The greens are definitely getting faster, and it's firming up. Iit's a good position. Four behind going into any U.S. Open third round is a good place."

(Editing by Pritha Sarkar)