(Reuters) - The teenagers at the top of women's golf, Lydia Ko and Brooke Henderson, form a mutual admiration society and a burgeoning rivalry that will feature at the U.S. Women's Open starting on Thursday.

World number one Ko of New Zealand and second-ranked Canadian Henderson, one year younger at 18, have won the year's first two majors and tee off together in the first round of the Open at CordeValle in San Martin, California.

The Korean-born Ko won the Ana Inspiration in March and Henderson claimed her first major last month at the Women's PGA Championship.

Henderson followed up her major triumph by repeating as Portland Classic champion last week. Ko won the Walmart NW Arkansas Championship in her last tour start.

"Lydia Ko is amazing," Henderson told reporters when asked about the tour's youth movement. "She's an inspiration to all of us girls out here, and I think everyone in the world.

"She's done amazing things in her career and she's only a couple months older than I am. So I think everybody looks up

to her and thinks she's an inspiration. And we're just trying to kind of chase after her ... chase after our own dreams."

Ko, who won the 2012 Canadian Women's Open LPGA event at age 15 and last year at 17 became the youngest player ever to ascend to golf's world number one, is a fan of Henderson.

"To me, she's a really great player," Ko said about Henderson, a one-time aspiring ice hockey goalie. "We all know how amazing talent she has.

"She's an aggressive player, but at the same time she's smart. And I think she's one of the most confident putters out there, too."

Ko and Henderson are grouped with American world number four Lexi Thompson, a stately 21 years of age, representing the top rated players in the field at the year's third major in the absence of South Korean Park In-bee.

World number three Park, the 2008 U.S. Open champion, is out due to a persistent left thumb injury.

Nine former U.S. Open champions, including last year's winner, 19-year-old Korean Chun In-gee, are in the field.

Pak Se-ri, who launched the wave of Korean players on tour with her U.S. Women's Open victory in 1998, has been honoured with a special invitation to compete this week in what will likely be her final U.S. Open as she plans to retire following her 2016 campaign.

Another player sure to draw attention is 20-year-old Ariya Jutanugarn, who after missing 10 consecutive cuts last year blossomed in 2016 with three straight wins in May and two top-fives in the majors.

"I feel more confident (than) anytime I play U.S. Open," said the Thai, who missed the cut in her three previous chances. "I just want to have fun, enjoy."

Ko likes being in her high-powered grouping.

"I think a lot of people come out to watch Brooke, Lexi and I," said Ko, who has three wins and three runner-up finishes this season. "It's going to be really cool."

Henderson, who tied for fifth in last year's Open, said she was looking forward to playing alongside Ko.

"Of course, I'll always look up to her," said the Canadian. "She'll always have a lot of respect from me.

"But that doesn't mean I don't want to beat her."

(Writing by Larry Fine; Editing by Andrew Both)