Phatlum's first mistake of the day proved costly, an errant drive on the eighth leaving her no option but to hack out sideways from thick rough.
If she wins, Phatlum will become the second Thai golfer in three years to take the title, but is being pushed all the way by England's Georgia Hall, who birdied the 18th today to remain just one shot off the lead in this Ladies Professional Golf Association championship at an event which is the fourth of this year's five women's majors.
While first-round leader Minjee Lee and encountered problems down the stretch at a rainy Royal Lytham to give up two-shot leads on Friday, Pornanong played a steady hand and put her pink ball in all the right places - explicitly, out of the many bunkers that define the course.
Roared on by the large galleries in her home country, the 22-year-old Hall only took the lead for the first time after a 20-foot putt for birdie at the 16th hole and stayed steady to post 5-under 67.
Hall finished up with her only bogey of the day on the eighteenth but never looked likely to allow her lead to slip and joined Dame Laura Davies, Allison Nicholls and Karen Stupples as an English-born victor of the British Open.
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"To actually have that - luckily it was just a tap-in!" "I'll be back here hitting long irons while those kids will be hitting wedges". "I was so happy".
Hall also wowed the fans at Centurion in May alongside Charley Hull, the pair reaching the quarter-finals of the GolfSixes and earning a 1-1 draw with their male English counterparts Eddie Pepperell and Matt Wallace. Then, when Hall rolled in a 10-foot birdie putt at No. 13, they were tied for the first time since the first hole.
She insisted it was vital not to become overwhelmed by the occasion, having gone on to scoop more than £375,000 in prize money.
"I was very confident going into this event from a year ago (she finished 3rd then)".
"I finished third a year ago and that experience definitely helped today". Golf is a very mental sport, more than people even think. "I played well today; I putted great".
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"It was so nice to have so much support".
Pornanong Phatlum of Thailand plays a shot from a bunker on 12th green during day four of Ricoh Women's British Open at Royal Lytham & St. Annes on August 5, 2018 in Lytham St Annes, England.
Buoyed up by a supportive overnight text from Tom Lehman, who won the 1996 British Open at Lytham, Hall finished at 17-under 271, becoming the second English player to win the championship after Karen Stupples in 2004.
Hall, who received a check of $490,000, became the first English major victor since Karen Stupples won this event in 2004, and the fourth overall, along with Laura Davies and Alison Nicholas.
But instead of reflecting on the past, the former Oakmead College pupil hoped her triumph would inspire the next generation.
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She has already put some distance between numerous big names in women's golf.