Leading amateur golfer Lydia Ko is through to the quarter-finals of the United States girls' junior amateur championship at the Lake Merced Golf Club in California.

Ko, who qualified as No.2 seed, was quick into her work on Friday (NZT) when she dismissed Californian Hee Wook Choi 7 and 6 to match her winning margin from round one.

In round three, Ko battled to be all square through the opening five holes with Nicole Morales from New York.

But the world No.1 amateur soon found her rhythm and never fell behind again in the match.

The 15-year-old won the sixth hole to go 1 up, and then doubled her advantage on the 11th.

Morales was back in the match when Ko bogeyed the par 3 15th, but the New Zealander restored her advantage on the following hole with a birdie.

She held on to win by 2 and 1 and make the final eight players, where she will face Yueer Feng of Florida, who enjoyed a comfortable 6 and 4 win over Summar Roachell of Arkansas to book her place in the final eight.

"Everything overall today was really good," said Ko.

"I'm pretty happy. Before I play again, I'm going to hit a few driver shots and hopefully get it going in a straight line."

The quarter-final and semi-final matches are early on Saturday (NZT), with the 36-hole championship final the following day.

NZ Newswire