Smith shot a second successive five under 67 at Shoal Creek to forge a dominant advantage at 10 under overall after the Australian completed her round before a near three-hour delay for bad weather, which ultimately forced play to be halted early. She is four shots behind Smith, who is at 10 under 134 for the tournament.
Smith, 33, has never won before in 222 LPGA starts, and her best finish at the U.S. Women's Open was a tie for 46th in 2013, the only other year she has made the cut.
"It was tough because I started thinking about the outcome and tried to play good".
That put her in a two-hole playoff with Hyo-Joo Kim, who shot 5-under 67.
Oh Su-hyun (68) sits at 6 under, alongside Ariya Jutanugarn, who was 1 under through eight holes in her second round. Whether you agree with Azinger or not (and many on the Internet do not), Jutanugarn added some personal context during a Q&A this Monday morning.
Jutanugarn then flew the green with her approach to 18 while Kim's came up short and found the front-left bunker, from where she was unable to get her escape close to the hole and two-putted for bogey while her Thai opponent played a delightful pitch and converted from four feet to force sudden death.
Even though she was in a good mood after the third round, Jutanugarn admitted she was exhausted.
"Yeah. So my went back to my old clubs I have been using for a couple of years, I guess", said Smith. She made seven pars in a row before back-to-back bogeys on Nos. Kim was on the green in two, while Jutanugarn pulled her second shot into a bunker, and both walked away with a par.
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She is also the first multiple victor of the 2018 LPGA Tour season, having already won the Kingsmill Championship.
For Jutanargarn, its a chance for her ninth LPGA title, after winning at Kingsmill just two weeks ago, and her second major crown, after her 2016 Women's British Open win.
Inbee Park, who now tops the world rankings, was the ninth player to finish under par, with England's Charley Hull and 2014 US Open champion Michelle Wie closing on level par for the week.
Hyo-Joo Kim hits out of the bunker to the 18th green during the second hole of a playoff in the final round of the U.S. Women's Open.
Two-time victor Inbee Park has an outside chance at getting back in contention. She was in the final group with Jutanugarn, but had a 78 on Sunday and tied for fifth.
Speaking after her win, Jutanugarn said: "I'm really honoured to join the list of players before me. Pretty hard, but I still can see the shot".
The Shoal Creek course has taken a lot of rain over the past few weeks - including almost an inch on Friday that forced the second-round delay - but still remained in relatively good shape.