Kostyuk, the Ukrainian star who has remained outspoken about the war in her homeland, was angered by a now-deleted post from the US Open's official Instagram account on Sunday evening in which she celebrated her straight-sets win over Maria Timofeeveva in the fourth round.
The graphic showed the Russian flag next to Timofeeveva's name, even though the 20-year-old plays as a neutral athlete on the WTA Tour.
“Today, after my victory over an athlete from the aggressor country, the Russian flag was published on the official source of one of the largest tennis tournaments in the world,” Kostyuk wrote.
“The sporting world continues to promote a murderous state and a state that uses its athletes as part of its propaganda.
“I call on media representatives, officials and the sports community to stop using the tennis court to promote ‘Russian peace’.”
Tennis has become one of the war's major sporting battlegrounds, with Kostyuk and other Ukrainian players regularly questioning why Russian and Belarusian players are still allowed to play in the WTA and ATP tournaments – even under a neutral flag.
Kostyuk used her post-match press conferences at the Australian Open this month as a platform to continue that conversation.
The 21-year-old took her argument to Instagram when she criticized the post from the US Open's official account.
“We've asked this question over and over again: What is the position of a neutral athlete?” “Because in reality it does not exist,” Kostyuk wrote.
“These athletes have said time and time again that everyone knows the countries they represent.
“Their government keeps saying these are their athletes and the world knows it.
“The international media wrote in their publications that these are Russian and Belarusian athletes.
“Many athletes are raising their flag on social media.”
Kostyuk defeated two Russian-born players en route to her first quarter-final berth at a Grand Slam.
She will face American Coco Gauff, ranked fourth in the world, in the quarter-finals on Tuesday.
If she defeats Kostyuk Gauff, she could face the Belarusian-born player and defending Australian Open champion, Aryna Sabalenka, in the semi-finals.
Kostyuk said last year that she had “no respect” for Sabalenka, after she refused to shake her hand after their match at Roland Garros.
“Just to deny her responsibility to have an opinion about the most important things in the world — I can't respect that,” Kostyuk said after the game.
Sabalenka responded to those comments by saying: “I have said many times, no one – Russian or Belarusian athletes – supports the war. Of course I will not support war. “If we could stop it, we would, but it is not in our hands.” – Callum Dick
8.30am: Alcaraz headlines Day 9 of the men's events
Carlos Alcaraz will face unseeded Serbian Miomir Kecmanovic in his quest to reach the quarter-finals of the Australian Open on Monday.
The second-seeded Spaniard has yet to be fully tested in Melbourne but has shown glimpses of his best.
Speaking after Saturday's third-round match, during which Chinese teenager Shang Junqing was forced to withdraw after trailing by two sets, Alcaraz said he was “feeling great.”
“Maybe seven, eight (out of ten),” the 20-year-old said, giving himself marks.
“It's a high note. But that's how I feel.”
“I think I'm getting better every day. Every match I play, I feel better. I'm moving, I'm hitting the ball and of course I'm getting used to this court as well. The Australian Open has become a shootout for first place, and defending champion Novak Djokovic has already set a milestone, losing three matches Only in his fourth-round match on Sunday.
Alcaraz, a two-time Grand Slam champion, knows he will have to raise his game to new heights to prevent the top seed from winning a 25th Grand Slam title in Melbourne, where the Spaniard's previous best performance was in the third round.
German Alexander Zverev, seeded sixth, will meet Britain's Cameron Norrie, seeded 19th, in a match that could be a long-shot, while Daniil Medvedev, seeded third, will face Portuguese Nuno Borges, seeded 69th.
Originally published as Australian Open 2024: Day 9 live scores, schedule and standings: Marta Kostyuk fuming over deleted tweet