Lexi Thompson has shown all class after a controversial rule decision cost her the lead at the LPGA's first major of the year yesterday.

A television viewer's email alerted officials to a day-old rules violation by Thompson for improper ball placement. Her three-shot lead on the 12th hole was wiped out a four-shot penalty as she tried to hole back tears.

Thompson survived the shock, and she forced a playoff with three gutsy birdies that had the Dinah Shore Course crowd on its feet.

But So Yeon Ryu managed to take advantage of the break created by Thompson's extraordinary penalty.

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Ryu birdied the playoff hole to win after Thompson was blindsided for an infraction she had accidentally committed 24 hours earlier.

Despite the drama, Thompson posted a classy reponse on Instagram this morning.

"Let me start by congratulating Seo Yun Ryu on her playoff victory yesterday at the ANA Inspiration. She fought extremely hard and played amazing golf," she wrote.

"I don't want anything that happened yesterday to take away from Seo Yun's victory. The LPGA rules officials made a judgment call at the moment, and we as professional golfers must accept it, no matter how painful it is. What happened was not intentional at all, I would never do that purposely. And I hope everyone knows that.

"I want to thank the amazing fans out on the course for all their support. Their cheering helped get me through those last 6 holes. Hearing all the fans cheer me on after every shot, going to every tee, truly brought tears to my eyes every time.

"I also want to thank the fans around the world who have reached out to me to offer their support, and for being there for me in this difficult situation. Your support means so much to me ....... I'm so blessed to have you all as my fans. I love you all and the game of golf, and I look forward to seeing all of you very soon."

Thompson had to settle for second place, a check worth roughly $155,000 less - and widespread sympathy as the latest golfer to get blindsided in a major by the peccadillos of this fussy, fastidious sport.

"It's great to have the fan base that I do, and they really got me through the whole round," Thompson told media yesterday. "It's unfortunate what happened. I did not mean that at all. I didn't realise I did that. I fought strong through the finish, and it was great to see the fans behind me."

The fans on the Dinah Shore Course largely shared the mixture of bewilderment and anger expressed by Thompson's fellow golfers and most viewers online.

BAFFLING RULE

The 22-year-old U.S. Olympian is the third major contender to be hit with a perplexing penalty for a violation of golf's stringent rules in the past year. The decision reignited the debate about this sport's baffling willingness to allow armchair refereeing - and renewed hope for common-sense rule changes that could be adopted as early as 2019.

"Viewers at home should not be officials wearing stripes," Tiger Woods tweeted. "Let's go @Lexi, win this thing anyway."

Thompson was penalized for marking and moving her ball less than an inch before a 1-foot putt on the 17th green during her third round at Mission Hills Country Club on Saturday. After putting down a marker and picking up the ball, Thompson swiftly put it back - but not precisely in the same place, video review showed.

The minor action went unnoticed live, but a television viewer spotted it and emailed tour officials while Thompson was playing the front nine on her final round with Suzann Pettersen on Sunday.

After nearly two hours of agonizing over video of the moment, LPGA Tour rules official Sue Witters got the unpleasant job of informing Thompson about her penalty after she left the 12th green.

"Is this a joke?" Thompson asked.

Even Witters understands the prevailing opinion about the ruling.

"Sure, but what's my choice?" she asked. "(Allow) a violation in the rules, and then it would be the opposite story: 'Oh, they knew. Why didn't they do anything about it?' I can't go to bed tonight knowing that I let a rule slide.

"It's a hard thing to do, and it made me sick, to be honest with you."

Sickening things have happened with regularity in majors lately.

At the men's U.S. Open at Oakmont last year, Dustin Johnson won despite playing the last seven holes without knowing his score. His ball had moved slightly while he lined up a putt on the fifth green, the USGA eventually hit him with a one-shot penalty after his round.

A few weeks after that at the U.S. Women's Open , Anna Nordqvist got a delayed two-stroke penalty during a three-hole playoff for accidentally touching the sand with her club in a bunker when a television cameraman spotted it. Brittany Lang won by - you guessed it - two shots.

Perhaps all of this drama only underlines the importance of what could soon happen to that centuries-old rulebook.

The two governing bodies of golf released a draft of modernized rules last month, attempting to simplify the arcana. The numerous proposed changes would eliminate some of the punishment for inadvertent mistakes and freak occurrences that don't actually give an advantage to a competitor.

But those changes haven't been adopted yet, and Thompson was left with a crushing penalty for an action that easily could have been ignored under a more pragmatic interpretation of intent - a "reasonable judgment standard," under the proposed rules.

Instead of losing her composure after the ruling, Thompson incredibly birdied the next hole, burying a 25-foot putt. She made two more birdies and a bogey down the stretch, but missed a 15-foot eagle putt to win it on the last hole of regulation.

Ryu nearly put her playoff approach shot into the water, but got up and down with a 6-foot birdie putt to win. Her celebration was understandably muted, but she still took the traditional leap into Poppie's Pond.

A few yards away, Thompson embraced her family and later signed dozens of autographs. She was shaken, but not deterred.

"Every day is a learning process," Thompson said. "I wasn't expecting what happened today, but ... it happens, and I'll learn from it and hopefully do better."