June 10 (Reuters) - Wyndham Clark is frustrated with the state of his game and just looking to produce four solid rounds when he starts his U.S. Open defence this week at Pinehurst Resort in North Carolina.

The American was the major tournament's surprise winner last year when he held off Rory McIlroy on the back nine for a one-stroke victory at Los Angeles Country Club.

Despite winning February's Pebble Beach Pro-Am, which was cut short after 54 holes due to weather, he is not brimming with confidence following missed cuts in three of his last five starts including the Masters and PGA Championship.

"I haven't been playing my best golf," Clark told reporters on Monday.

"It's been kind of a tough stretch these last few weeks. Really I'm trying to gain some momentum for the rest of the season. I know that maybe sounds like low expectations for the week, but honestly I'd love to just gain some momentum.

"I'd really like to hit some good shots, have some really good up and downs, make some key putts throughout the week, and play four solid rounds."

He said his frustration level is higher than normal in part because the excellent form he has in practice has not shown up when he needs it most.

"It's kind of a bummer," he said.

"It's the great thing about golf: there's always another week. I've got to believe that good golf is around the corner."

The U.S. Open is known as golf's toughest test and the speedy greens at Pinehurst No. 2 will challenge even the world's best, Clark said after Monday's practice round.

"The greens are extremely fast and penal. You hit it on the green, the hole is not done. I was just amazed how fast the greens are," he said.

"If they get any firmer and faster they'd be borderline. They already are borderline."