Ji opens LPGA Match-Play defence with victory over Castren
Ji, who beat Japan's Ayaka Furue in last year's final at Shadow Creek in Las Vegas, Nevada, gained the upper hand on Finland's Castren with a par to win the second hole and was 3-up after winning the 12th with a par.
Castren cut the deficit with a par to win the 14th, but a par was enough for Ji to win the 15th and when they split the par-five 16th the match was hers.
"So I didn't have the best shot today, but I think luck came with it," Ji said. "I don't think my opponent had the best day either, so that helped ease the process a little bit."
Going up early was a big help on a windy day at a challenging course, Ji said.
"When I have the lead in the beginning, I think I can definitely not lose the lead," said the 37-year-old, whose six LPGA titles include the 2009 US Women's Open. "So even though I didn't have the best shot today, I did my best to get a save.
"Shadow Creek definitely isn't an easy course. Every hole is very complicated so you have to think a lot about that."
Vu, who won her first major title at the Chevron Championship in April, is the highest-ranked player in the 64-strong field at fourth in the world.
She never trailed on the way to a 4&3 victory over fellow American Lauren Hartlage, winning four straight holes from the 12th through the 15th to pull away after Hartlege had tied it up with a birdie to win the 11th.
"Being number one seed, everybody says it's a curse but I was just trying to forget about that and just play one shot at a time," said Vu, who rolled in a long birdie putt at the tough par-four 15th to seal the win.
Round-robin play, in 16 groups of four, continues through Friday, with 16 players advancing to the weekend knockout rounds.
There were dominant 5&4 wins for Thailand's Pornanong Phatlum, American Lauren Coughlin and Aussie Karis Davidson.
Pornanong defeated Mexico's Gaby Lopez while Coughlin beat fellow American Andrea Lee and Davidson downed China's Lin Xiyu.
Ecuador's Daniela Darquea, 3-down through 10 holes against American Alisen Corpuz, won the next two holes with birdies as she turned it around for a 1-up win.
"It was really fun," said Darquea, who honed her match play skills as an amateur focused on the Copa Los Andes.
"Playing again match play after so many years, it's special, it's important. So many good memories," she said.
France's Perrine Delacour was 1-down with two holes to play but won the final two to seize a 1-up victory over India's Aditi Ashok - taking the lead for the first time at the final hole.
