American Stefan Kozlov and Argentina's Guido Andreozzi battle on Day One of US Open qualifying in the shadow of Arthur Ashe Stadium.

Shapovalov, Opelka Off To Winning Starts At US Open Qualifying

Opelka one of seven Americans to advance to Round 2

Five #NextGenATP stars moved into the second round on an oppressively hot and humid Day One of US Open qualifying.

Conditioning and stamina were put to the test as Akira Santillan battled back for a 3-6, 6-3, 6-4 win over Sebastian Ofner. The Aussie fought through heat exhaustion and cramps for a two-hour and two-minute victory, converting three of 13 break chances. Santillan called for the trainer at 4-all in the third set and proceeded to claim the decisive break immediately out of the medical timeout. He fell to his knees after serving out the match a game later.

In other action, 11th seed Casper Ruud breezed past Dmitry Popko 6-4, 6-0 in 64 minutes, while Canadians Denis Shapovalov and Felix Auger-Aliassime kicked off the day with straight-set wins themselves. Reilly Opelka was the lone #NextGenATP American to claim victory on home soil, edging countryman Alexander Sarkissian 6-7(5), 6-3, 7-6(2).

"It doesn't feel as quick as it was, when you're out there in the heat," Ruud told ATPWorldTour.com. "They have this new shot clock out there which counts the seconds between serves, which is a bit stressful as well. I'm happy to be able to deliver a good match. I was tight in the beginning but played well and was able to win the second set pretty quickly.

"It's a great event. They have the biggest stadium in the world here. It's a great atmosphere and I'm looking forward to the next match."

Competing on adjacent courts, 17-year-old Auger-Aliassime prevailed 6-3, 6-3 over Hiroki Moriya, while 18-year-old Shapovalov downed Denis Kudla 6-3, 6-4, just minutes later. Shapovalov enters in strong form following a semi-final run at the Coupe Rogers two weeks ago. A Next Gen ATP Finals contender (No. 5 in the Emirates ATP Race To Milan), he is bidding to qualify for his first US Open main draw.

Meanwhile, the upset of the day was served by American JC Aragone, who stunned fifth seed Marco Cecchinato 4-6, 6-2, 6-4 in two hours and one minute. The University of Virginia standout scored the biggest win of his young career over the World No. 101 in just his first tour-level qualifying match.

"I kept telling myself to go out there and have fun," Aragone said. "Just rip the ball and there's no pressure. That last game I was trying to hit everything as hard as I could. Luckily it went in.

"This is just amazing especially since I came here on Saturday and I wasn't even in the draw. I got a last-minute wild card. I wasn't in the tournament and then I'm the last guy in. I got the opportunity to play and I made the most of it."

Other Americans advancing to the second round include Sekou Bangoura, Bradley Klahn, Dennis Novikov, Mackenzie McDonald and Christian Harrison.

Top seed Leonardo Mayer and fourth seed Sergiy Stakhovsky - two of three Top 100 players in the qualifying draw (also Shapovalov) - advanced as well. Mayer, who is coming off his second ATP World Tour title in Hamburg, defeated Maxime Janvier 6-4, 6-4. Stakhovsky is also on a roll after clinching the ATP Challenger Tour crown in Portoroz, Slovenia, and the Ukrainian blasted past Alexey Vatutin 6-2, 6-1 in just 65 minutes. He fired seven aces and saved all five break points faced.