The first LPGA major of the year has descended into controversy, after a TV viewer alerted officials to an indiscretion by leader Lexi Thompson from the previous round, prompting a four-stroke penalty.

Thompson was three shots clear in the lead after the 12th hole of her final round at Mission Hills Country Club, when officials informed her she had been penalised two strokes for incorrectly replacing her ball on the 17th green the day before.

The 22-year-old American was then penalised an additional two strokes for subsequently signing an incorrect card.

She eventually forced a playoff, but lost to Korean So Yeon Ryu. When quizzed about the contentious incident after her round, Thompson said she “didn’t realise [she] did that”.

Lexi Thompson was assessed a 4-shot penalty for an incorrect marking of a ball and signing an incorrect card yesterday at #ANAInspiration pic.twitter.com/6pNJ5haql9

— Golf Channel (@GolfChannel) April 3, 2017

Officials missed the incredibly minor indiscretion on the day, and were not alerted to it until a television viewer informed competition organisers from home.

“Is this a joke right now?” Thompson asked officials, before regaining her composure to continue her push for victory despite the setback.

Thompson fought to control her emotions after a brilliant second shot on the 18th, as the crowd rallied behind her and chanted her name.

Here is Lexi Thompson being informed of the penalties: "Is this a joke?" https://t.co/bzpmJzh40f pic.twitter.com/gHRFPUlT9C

— Cork Gaines, Ph.D. (@CorkGaines) April 3, 2017

Despite Thompson taking momentum with her to the playoff hole, Ryu birdied the last to claim victory.

“[The fans] got me through the whole round,” Thompson said after the playoff.

“I felt strong through the finish and it felt great to see the fans behind me.

“I learned a lot about myself and how much fight I do have in me.

“I wasn’t expecting what happened today, but it is what it is and I’ll learn from it. I fought strong and that’s all I could do.”

It is not the first time an eagle-eyed TV viewer has influenced golf officials, with Tiger Woods the victim during the 2013 Masters, when an incorrect drop cost him two penalty strokes.

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

— Tiger Woods (@TigerWoods) April 3, 2017

Thompson’s playing partner, Suzann Pettersen, fell one stroke short of the playoff, alongside Australia’s Minjee Lee, and said she wanted the American to go out and win the playoff after showing such mental fortitude out on the course.

“I felt awfully bad for her,” the Norwegian said.

“I thought she took it with the character she is – she fought back with birdies. It’s just true class from Lexi, taking that.”