COSTA MESA CITY CHAMPIONSHIP:

COSTA MESA — Marcus Sostak believes the key to success in tournament play is convincing himself he is merely playing a round of golf with good friends, with nothing more at stake than friendly bragging rights.

Of course it helps support this scenario when he is, in fact, paired with familiar faces, as he was Saturday in the opening round of the Costa Mesa City Championships at the Mesa Linda course of the Costa Mesa Golf & Country Club.

Sostak, a former Estancia High standout who has transferred from the University of Oregon to Orange Coast College, where he plans to play next spring, shot four-under-par 66 Saturday in a group that included Estancia High products Andy Rothman and Jeff Ohland.

Heading into today’s final round, on the Los Lagos course, Sostak, a three-time Golden West League individual champion while at Estancia, is in a six-way tie for third in the Championship Flight, four shots behind Mark Harding of Redondo Beach, who shot a career-best 62 Saturday.

Harding, a perennial participant in the tournament, now in its 36th year, made nine birdies to emerge as the clear favorite. His best finish at the Costa Mesa City Championship is 12th or 13th, he said, but he said he typically plays better on the longer Los Lagos course (par 72).

Big Canyon Country Club men’s champion Will Tipton, who lost in a playoff to settle for runner-up honors in this tournament a year ago, is tied for second with Tustin resident Karl Katnik, after both shot 65.

Also four shots back of Harding — 48, a former national outdoor racquetball champion (1981, 1982 and 1984) who took up golf after a string of knee surgeries prompted his departure from the racquetball court — are Mike Carpenter, a Newport Beach resident who won the title in 2001, Eric Vallely, a former Newport Harbor High basketball and volleyball standout who lives in Costa Mesa, and former UC Irvine golfer Sean Shahi.

Huntington Beach resident Steve Oppegard and Rancho Santa Margarita resident Jason Bittick also shot 66 Saturday.

Defending champion Chris Hirahatake shot 72.

Sostak, who grew up playing at the Costa Mesa Golf & Country Club, said he sank a 35-foot birdie putt on No. 17 to post one of the best rounds he has ever recorded on the Mesa Linda course.

Sostak said he has not played too many tournaments since leaving Oregon, but he did win the Cal State San Marcos Invitational last winter. It’s his third time in the Championship Flight at the Costa Mesa City Championship.

“I love it here, just going out and having fun,” he said. “Last year I was 13th [71-73—144].”

When asked if he had a target number in mind for Sunday’s round, Sostak revealed his have-fun approach.

“I try not to expect things when I play and just take it one shot at a time,” Sostak said. “If you have expectations, it just ruins you, I think. I’m just going to try to have fun [today]. I’m just going to pretend I’m playing a practice round with my friends. That’s how I play best.”

Harding said he was able to play his best by avoiding the dangerous putting zones above the holes, created by the difficult pin placements the tournament is known for.

“The pins are set so tough, if you’re not always underneath the hole, you’re putting defensively,” Harding said. “Today, my yardages were right on and I was knocking it close, but always underneath the hole. It was almost a fluke how I was landing underneath the hole all day long, which made it one of the easiest rounds I’ve ever played.”

His previous low round was 65 several times on par-72 courses.

“I never had to make any lengthy putts and I chipped in once,” Harding said.

Kevin Bodlovich, Nicholas Ellis, David Lyons, Henry Huang and Bryan Harris all shot 67 Saturday to remain in contention, a group that also includes Tyler Annear, Mel Collins Jr., Brandon Van Goethen and Allen Geiberger, the son of former PGA Tour player Al Geiberger Jr., who all shot 68.

Steve Trivelpiece of Irvine leads Flight 1 with a net round of 68 (their gross score, minus their handicap), followed by Costa Mesa residents Brendan McTeggart and Roger Turk, both of whom shot net 70.

Laguna Niguel resident Scott Harris shot net 63 to lead Flight 2, five shots ahead of Costa Mesa resident Doug Hutchinson (net 68).

Also contending in Flight 2, are Gregory Thralls (net 69), and Jim Golotko, Fred Cook, Kevin Sverduk and Paul Bush (all net 70), as well as Andre Brysha and Costa Mesa residents Mark Miller and Joe Vierra (all net 71).

Mark Simmons (net 65), Garry Newbrough (net 66), Dennis Noren (net 67) and Ray Aldrete (net 68), as well as Costa Mesa resident Kyle Raymond (net 70) are contenders in the Senior Flight.

Golfers tee off beginning at 6 a.m. today. The leaders in the Championship Flight is scheduled to tee off at 12:10 p.m. The Championship Flight will play on Los Lagos today, while those who played Los Lagos Saturday will play Mesa Linda today.

BARRY FAULKNER may be reached at (714) 966-4615 or at barry.faulkner@latimes.com.