Considering it was the biggest win of her life, Lea Nilsson did not have the reaction you would expect.

When the 17-year-old Swede watched her opponent’s last backhand sail wide here at the US Open Junior Championships, Nilsson’s reaction was so small you could well have missed it.

She made a small fist pump toward her coaches' box, and then calmly walked up to net to shake hands.

It was a result that rocked the girls' draw, yet Nilsson acted like it was the end of a casual Wednesday morning practice session.

In a match played on the second-biggest court here, junior world No. 82 Nilsson stunned Julieta Pareja, the top-ranked girl on the planet, 6-4 3-6 6-3.

It is the first time since 1996 that no American girl has reached quarter-finals in the singles draw at the US Open.

Qualifier Nilsson won 72 per cent of first serve points and broke Pareja, a finalist at the Junior Championships, Wimbledon in July, four times.

“There were a lot of emotions, this was a big deal,” Nilsson said. “But on the outside I maybe seemed less happy, but on the inside I was very excited.”

In defeating Pareja, Nilsson kept an unlikely run going here in New York. She had mere hours to prepare for her first qualifying match on 29 August, and won her first round match in a tiebreak. She had spent the past week playing J300 Repentigny in Canada.

“I think my flight was 7am, and we landed and I got my accreditation and then went right to the courts,” Nilsson said with a smile. “It wasn’t the best preparation.”

But Nilsson found a way to win, and then she kept on winning, three more times until her showdown with Pareja.

After a strong first set, Nilsson let Pareja back into the match and went to the third set not knowing if she would be able to close out the heavily-favored American.

“I think I started really solidly and strong, I got good rhythm,” Nilsson said. “And the second set I stood still a little. I didn't get the rhythm with my backhand. And she played really, really well.

“Then in the third set, I just fought on every point.”

Nilsson, from Stockholm, trains at Salk Tennisklubb and has had success at junior tournaments this year, reaching the third round at a trio of J300 events, but has struggled at Grand Slams. She reached the second round at the Australian Open and Wimbledon and lost in the first round at Roland Garros.

But her game is suited for hard courts: she has a big serve and a big forehand that she used to repeatedly keep Pareja off balance.

“To keep the tempo high and intensity in the legs is the most important thing for me,” Nilsson said. “If I play against someone who has a lower tempo than me, I try to keep my level and play my game.

"When I have good intensity with my legs, moving well, I usually hit the ball better, more fluid and not scared.”

Nilsson certainly was not scared of anything on Wednesday, and now faces Spanish No. 8 seed Charo Esquiva Banuls for a spot in the semis.

First, though, Nilsson needed to do something important after her interviews.

“I haven’t checked my phone yet since the match,” Nilsson said with a laugh. "So maybe I got a few messages.”

A full list of results from the US Open Junior Championships is available here