Coleman Wong was five years old and living in Hong Kong when a sibling’s illness changed his life forever.

Elana Wong, aged 7, was supposed to have a tennis lesson at Victoria Park Tennis Courts when that morning she came down with a fever.

Coleman, a self-described “pretty chubby” kid, was told by his parents that he should take the lesson instead, so as not to waste the money.

“And I did I found I had a little talent for it,” he said with a big smile (Wong doesn’t seem to say anything without smiling at the end). “It was kind of funny, but the timing for hitting the ball was always there, and since that first day it was always fun.”

Elana Wong’s high temperature has led to some pretty amazing things for her brother. On Thursday in New York, Coleman, now 18, advanced to his first-ever US Open Junior Championships semi-final with an extremely tight 6-2 6-7 6-4 triumph over No. 3 seed Kilian Feldbausch of Switzerland.

Wong, the No. 9 seed, is already the owner of two Junior Grand Slam doubles titles, but admitted a singles crown, the first-ever for a Hong Kong boy, would be a whole different level.

”My focus is always singles, and doubles is a bonus, and I am training to get better in singles,” said Wong, who featured in a behind-the-scenes ITF video at J1 Roehampton and the Junior Championships, Wimbledon in July. “To win for Hong Kong, there aren’t many top tennis players there, so it would be so great to win. Maybe it would make more people play.”

Wong’s victory over Feldbausch was made sweeter since the Swiss beat Wong in the first round at the Roland Garros Junior Championships this year.

The two had many long rallies throughout the match, and after Feldbausch snagged the second set, it looked like he had all the momentum.

But Wong and his 125 miles-per-hour serve was too tough; at 5-4, 15-30 in the final game, Wong blasted two aces, then smashed a service winner on match point.

“I was just thinking (at 15-30), ‘how can I win this point, what can I do to win this point, and what should I do the first two shots?' Then, guess what? Two aces,” he said, grinning. “I just felt like my serve helped me a lot.”

In the semi-finals, Wong will face No. 2 seed Gilles Arnaud Bailly of Belgium, who also went the distance with American Kyle Kang, with Bailly advancing following a 6-4 3-6, 6-1 victory. 

The other semi should be a cracklingly good one as well, with top seed Daniel Vallejo, a straight sets winner against Hynek Barton, looking to become Paraguay’s first Junior Grand Slam winner since 1992, facing off against No. 5 seed Martin Landaluce of Spain, who outlasted Belgium’s Alexander Blockx in three sets.

“I didn’t play well today so I’m so happy I was able to get through,” said Landaluce, who reached the last four at the Junior Championships, Wimbledon in July.

On the girls' side, No.2 seed Lucie Havlickova scored an impressive straight-sets win over American Clervie Ngounoue, 6-4 6-2, but after three matches played on Wednesday, she didn’t feel her best Thursday morning.

“When I woke up I thought I was dying,” Havlickova said after advancing to her first US Open semi-final. “My arms hurt, my legs hurt, everything hurt. I was thinking like ‘OK, I’ll retire, she’ll have a walkover, and I go back to sleep'.”

But a session with her physio got the Czech star back in playing shape, and she looked very solid against Ngounoue, the last American left in the singles draw. Serving at 5-4 in the first set, she survived a 24-point game, saving three break points to win the set.

“I was nervous, and my backhand was so bad, every other one was sailing into the fence,” she said. “But my serve saved me, so thank you, serve!”

Havlickova next meets someone she knows quite well, No. 7 seed Diana Shnaider, who advanced with a victory over Great Britain’s Ranah Stoiber.

The other girls’ semi-final will pit Victoria Mboko of Canada, who improbably rallied from 3-5 down in the third set to defeat No.4 seed Celine Naef of Switzerland, against No. 10 seed Alex Eala of the Philippines, who easily handled Mirra Andreeva, 6-4 6-0.

Eala is bidding to become the first girl from her country to win a Junior Grand Slam, and was again cheered on by a dozen or so local Filipinos in the crowd.

There are three players still alive in both singles and doubles draws; Landaluce and partner Pedro Rodenas are in the boys’ doubles quarter-finals, while Havlickova and partner Shnaider will face off in singles on Friday before teaming up in doubles.

“It's the exact same thing I had at Roland Garros,” Havlickova said. “I beat Sara Bejlek in the semi-finals, and then we had doubles together. After I beat her, I was scared she was going to be mad, but she didn’t seem mad at all. After we won the doubles final she said ‘I was so annoyed at you, but I wanted to win the doubles!”

"So Diana and I will be fine, whoever wins, we’ll still be happy playing doubles together.”