Chris Williams

In a 36-hole marathon at Exmoor Country Club, Williams shot 6-under 66 in his morning round, backed it up with a 5-under 67 in the afternoon, and won the 72-hole stroke-play segment of the championship with a 17-under par total - one stroke better than his medalist performance last year at North Shore Country Club in Glenview.

"I'm pretty happy with myself," said Williams, the top-ranked player remaining in the field after the 36-hole cut. "I've been working hard all year and it finally paid off at one of the biggest tournaments in the world."

Williams and 15 other players will compete in the championship's Sweet 16 match play portion of the event, which begins Friday and concludes with a final match Saturday afternoon with whichever two players remain standing.

Runner-up Lorens Chan, 18, of Honolulu, the youngest player to make it to the Sweet 16, was paired with Williams all day Thursday. Chan actually led the tournament for awhile on the front nine of their second round - until Williams heated up. Chan, who finished at 15-under after rounds of 68-67, was impressed with Williams - to say the least.

"I got a front row ticket to see one of the top-ranked amateurs in the world," said Chan, who will be a freshman at Pac-12 rival UCLA and is playing his first serious amateur golf schedule on the mainland this summer. "Now I see the difference between junior golf and the top-ranked amateurs in the world. It was fun to watch."

Chan, whose mother, Linda, has been caddying for him all week, said earning a spot in the Sweet 16 "will kind of jump start my college career. This is one of the biggest amateur tournaments in the world, so playing well here will give me a lot of confidence."

Finishing in a three-way tie for third was No. 7-ranked Kentucky native Justin Thomas of Alabama, one of the more highly decorated college players of 2012. The Haskins and Nicklaus award winner recorded an even par 72 in his first round, but shot his third and fourth straight rounds in the 60s with 69-67 on Thursday.

"I'm honored to make it to match play," said Thomas. "You play well and everything is going to take care of itself. It's going to be fun. I'm pumped." Jeff Kang and second-round leader Abraham Ancer also finished T-3.

Other Sweet 16 qualifiers ranked highly by the Scratch Players were No. 14-ranked Zac Blair of BYU (ninth place at 10-under), No. 17 Peter Williamson (T-6 at 11-under), No. 18 Jordan Russell of Texas (T-10 at 9-under).

An eight-man playoff for four spots in the Sweet 16 produced top contestants as well as a notable controversy.

Qualifying via their play were No. 15-ranked Cheng-Tsung Pan - Williams' teammate at Washington and the Western Amateur medalist in 2009 and '10 - Canada's Mackenzie Hughes, and North Florida's Sean Dale. Richard Lamb of South Bend, Ind., and Pedro Figueiredo of Portugal received spots in the field after two Australian players were disqualified.

Daniel Nisbet, 20, of Caboolture qualified for the Sweet 16 by finishing stroke play at 11-under par but was disqualified when he caddied in the playoff for fellow countryman Matthew Stieger - a violation of the conditions of the competition. Stieger, 21, also was disqualified.

The rule sheet, which is distributed to all players when they register for the championship, says: "Players that have been eliminated (missed the cut or lost a match) from the tournament are allowed to caddie for a player still in the field. Players still competing for the Championship are not allowed to caddie for another player in the field. Penalty: Disqualification of both players."

Rules officials observed Nisbet caddying for Stieger on the fourth playoff hole (No. 18) as Stieger competed against Figueiredo for the final spot, according to Vince Pellegrino, vice president of tournaments for the Western Golf Association.

With Nisbet and Stieger disqualified, Figueiredo and Lamb, who had been eliminated on the third playoff hole (No. 10), took their places in the Sweet 16.

Pellegrino said he was not aware of any other players in recent history having been disqualified for violation of the caddie rule. It is common to see players who have been eliminated from the competition caddying for teammates or friends.

"It's an unfortunate circumstance, but it's clearly stated in the rules," Pellegrino said.