Her troubles coupled with some astonishing long-range putting from Kim produced a dramatic finale which had appeared unlikely halfway through the final round, and the pair finished 72 holes tied on 11 under when Kim returned a 67 while Jutanugarn bogeyed the final two holes to sign for a 73.
She righted the ship in the playoff, which went to a sudden-death format after the two-hole aggregate start failed to produce a victor.
Going to the 18th for the final playoff hole both Kim and Jutanugarn missed the green. But the Thai star survived long enough to defeat Hyo Joo Kim on the fourth playoff hole Sunday at the U.S. Women's Open.
She made a almost flawless bunker shot to within a foot of the cup on the tournament-clinching hole, beating South Korea's Hyo-Joo Kim, who shot a 5-under 67 in the final round to force the playoff. Half the field completed the second round yesterday morning.
Her cushion had been whittled down to two shots when she bogeyed the 12th, minutes after Kim had holed a 40-footer putt, and when the Korean sank an even longer putt, 50 feet from off the green at the 15th, Ariya's lead was down to one. Many players are yet to complete their final round.
"I think they could have been really good if I had taken the time to do it in the offseason". So we had a really good lesson before Kingsmill and then I actually missed the cut in Kingsmill, but felt like I played really well.More news: Asus ROG gaming smartphone with 90Hz display refresh rate, overclocked CPU launched
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"That back (nine) got me a lot", said Ariya, who won her first major at the 2016 Women's British Open.
Smith opened the day in a three-way tie atop the leaderboard with Thailand's Ariya Jutanugarn and Korean Jeongeun Lee.
"I hit a couple squirrelly shots (late in the round), but made some nice up-and-downs".
She has had a particular liking for Shoal Creek's back nine, playing it in 10-under.
"It did not really enter my mind at the start of the day that I was going to come that close, so I just focused on playing my game", said Kim. "Hopefully, tomorrow I can go out with a little bit more confidence".
Smith now sits four shorts clear of fellow Australian Oh Su-Hyun.
"After you have a seven-shot lead and end up with you having to go to a playoff, I had no expectations", she said. "But I mean ... if you hit your irons well because the course is soft, you can get access to the pins and trust that you have good numbers, it's definitely there so you can go low".