The long wait to play the quarter-final matches on a rainy Thursday at the US Open Junior Championships didn't end with any sunshine, but for the underdogs the results couldn't have been much brighter.

Robin Montgomery and Sebastianna Scilipoti took out the top two seeds in the girls' singles draw on Thursday evening, and Jerome Kym shook up the boys' draw with victory over No. 2 seed and Wimbledon champion Samir Banerjee.

The rain, which caused a two-hour delay in the start of play, returned after just a few games of the first four singles matches on the schedule. The showers continued until after 6pm, leading to the cancellation of the day's doubles quarter-final matches, but when the courts dried the second time, the singles quarter-finals were completed under the lights at the Billie Jean King National Tennis Center.

Switzerland's Kym, the No. 8 seed, ended 17-year-old Banerjee's eight-match winning streak in Grand Slam junior events, coming from a set and a break down to post a 4-6 6-3 6-4 victory.

Kym trailed the American 6-4 3-1 before reeling off six straight games to take the second set, using his 16 aces to help keep Banerjee off balance and on the defensive. But serving while up 5-2 in the third set, Kym couldn't convert, and when Banerjee held for 5-4 and got to 30-all on Kym's second attempt to serve out the match, the pressure began to mount. Fittingly, an ace ended the match for Kym, who was not surprised by the drama at the end.

"I'm used to these situations," said the 18-year-old from Basel, who trains in Kitzbühel, Austria with coach Markus Hipfl. "It's normal that you are getting tight; it's the quarter-final of a slam. You have to remind yourself what to do: relax arm, towel, breathe two, three times. I did that at 5-4, the changeover. I tried to stay aggressive, loyal to my game plan." 

Kym, who reached the quarter-finals at Wimbledon, is now into his first Junior Grand Slam semi-final, where he will face No. 3 seed Daniel Rincon of Spain, who defeated No. 6 seed and Wimbledon finalist Victor Lilov of the United States 6-3 6-1 in just 57 minutes.

Rincon took a 2-0 lead before the rain sent him back to the junior players lounge for four hours, but that long wait to get back on court did nothing to stop his momentum.

"It was difficult, because you never know when you are going to start again," said the 18-year-old left-hander, who trains at the Rafael Nadal Academy in Mallorca. "I was having some food every hour, keeping the muscles active."

Rincon, who didn't face a break point in his quarter-final win, has taken inspiration from the run of 18-year-old compatriot Carlos Alcaraz in the men's singles here in New York.

"He's a great friend of mine," said Rincon, who will be playing in his first junior slam semi-final after reaching the quarter-finals this year at Roland Garros. "We used to play every single week together when we were 14, 15. So yeah, I'm really happy for him and it gives me some hope. He's doing great, doing amazing, something special, but I think I'm going to work even harder to get to where he is now."

American Robin Montgomery came into her match with top seed Victoria Jimenez Kasintseva of Andorra with revenge on her mind, having lost in their two previous meetings, including the 2020 Australian Open quarter-finals, after holding big leads.

When the rain came five games into the first set, Montgomery and Jimenez Kasintseva were on serve, but when they returned to the court four hours later, Montgomery took control, winning the first set, then dominating in the second for a 6-3 6-2 victory.

"I was telling myself if you go up, you have to figure out a way to win it," said Montgomery, who hit 20 winners to her fellow left-hander Jimenez Kasintseva's six. "Even before Australia, in Mexico, I was up a set and 5-2 and I lost that one as well. So I was a little bit shaky, but when I went up 5-1, I was like, ok, this is it. When she made it 5-2, she was getting a little more pep in her step and I was, no, no, no. You can't do this again. You have to serve this out Robin, I don't care what you do."

A forehand winner at 40-30 got the job done, with Montgomery earning a place in her first junior slam semi-final against unseeded Solana Sierra of Argentina. Sierra came from a set down for the third consecutive match, and for the second straight day won in a third set tie-break, beating No. 12 seed Elvina Kalieva of the United States 4-6 6-3 7-6(5).

Scilipoti added to the parade of surprise winners on Thursday night, taking out No. 2 seed Alexandra Eala of the Philippines 7-5 6-3. The 18-year-old from Switzerland will face No. 6 seed Kristina Dmitruk of Belarus, who beat unseeded Petra Marcinko of Croatia 6-3 3-6 6-3.

Top boys seed Juncheng Shang of China reached his second consecutive junior slam semi-final, defeating unseeded Pierre Bailly of Belgium 3-6 6-3 6-2 in the final match of the night. His opponent on Friday will be No. 11 seed Sascha Gueymard Wayenburg of France, who failed to win a game in the first set against unseeded Petr Nesterov of Bulgaria, but rebounded for a 0-6 6-4 6-4 win. Like Shang, Gueymard Wayenburg reached the semi-finals at Wimbledon this summer.

With the weather outlook for the weekend excellent, the singles semi-finals and two rounds of doubles are on the schedule for Friday, with the finals in singles and doubles set for Saturday.