A star at the University of Southern California, Ruffels, 19, survived the customary 36-hole final which lasted more than eight hours at Old Waverly Golf Club in West Point, Mississippi, and a rare caddie change.

In blazing Mississippi humidity, Ruffels switched caddies from her university's head golf coach Justin Silverstein to Mississippi State junior Blair Stockett, whose home club is Old Waverly, on the 31st hole.

Silverstein had to rush for a flight for a funeral.

Ruffels is the daughter of tennis star Ray Ruffels and sister of promising Tour golfer Ryan Ruffels.

She only took up golf at age 15 after tiring of competitive tennis.

In the first 18 holes on Sunday, Ruffels and Valenzuela traded the lead although it was the Victorian who had the momentum, at one stage holding a three-hole advantage.

The pair began the second 18 all square and were even until the 35th hole when Ruffels took the match by the scruff of the neck.

At the par-3 17th, she stuffed her 6-iron tee shot to within 10 feet and drained the birdie to enter the last hole with a one-up lead.

"She is one of the hardest workers I know; having this much success since taking up golf not that long ago shouldn't be possible but she's so gifted." – Ryan Ruffels

Valenzuela showed nerves of steel to hit a laser-like approach to four feet for a certain birdie, but Ruffels responded with an iron shot to within 15 feet and drained the putt to halve the hole and win the match.

"I'm still kind of speechless; this is my goal," an emotional Ruffels said moments after winning.

"I've worked so hard to get here. It was a great match and I am so glad I have the trophy."

Ruffels is the World No.52 amateur who made a big impression by winning last month's prestigious North & South Women's Amateur at Pinehurst.

Her older sibling Ryan, who plays on the PGA Tour's Latin America and Canadian Tours, was watching on TV and attempted to fly to Mississippi to caddie for his sister.

"That was wild; I'm lost for words," Ruffels, 21, told AAP on the phone.

"I tried everything to get there; I tried to fly in last night but it was impossible.

"Gabi wanted me to jump on the bag when her caddie had to leave.

"She is one of the hardest workers I know; having this much success since taking up golf not that long ago shouldn't be possible but she's so gifted."

- Evin Priest, Australian Associated Press