Early years

Marsak began learning to skate in 2009.^[1] In the 2018–19 season, he made his first appearance at the senior-level Ukrainian Championships.^[3]

2021–22 season

For most of the season, Marsak trained in Kyiv, coached by Dmytro Shkidchenko.^[4] On 24 February 2022, Russia launched a massive invasion of Ukraine. At the time, Marsak was 17 and living with his family in Kyiv. He initially remained in Ukraine but "not one day went by where there were no explosions." After three weeks, he fled with his sister to Poland.^[2]

In April, Marsak placed thirty-third at the 2022 World Junior Championships in Tallinn, Estonia.^[3]

2022–23 season

After spending some time in Latvia,^[5] Marsak went to Finland in June 2022 for a training camp led by Alina Mayer-Virtanen.^[2] In response to an appeal from the Ukrainian Figure Skating Federation, she and her husband, Valtter Virtanen, decided to help Marsak prepare for the season. In August, he began training at the couple's skating club, the Peurunka Skating Academy in Laukaa.^[5][6] Shkidchenko remained in Ukraine but continued to guide Marsak via Viber.^[2]

In September, Marsak placed ninth at the ISU Junior Grand Prix event in Latvia. He made his senior international debut in October, at the 2022 CS Finlandia Trophy. His first senior international medals, both bronze, came the following month, at the Volvo Open Cup in Latvia and the Tallinn Trophy in Estonia.^[3]

In January, Ukraine named Marsak to replace the injured Ivan Shmuratko at the 2023 European Championships in Espoo, Finland. He qualified to the final segment in seventeenth place after the short program, where he scored a personal best.^[7] Marsak placed twenty-second in the free skate and finished twenty-first overall. The day of the free program, Marsak had learned that his coach, Dmitri Shkidchenko had passed away.^[8]

He went on to compete at the 2023 World Junior Championships in Calgary, Alberta, where he finished fifteenth, scoring personal bests in the free skate and combined total segments of the competition.^[3]

Marsak made his World Championship debut at the 2023 World Championships in Saitama, Japan, where he placed twenty-fifth in the short program and did not advance to the free skate segment of the competition despite delivering a solid program. He closed his season by winning the gold medal at the 2023 Ukrainian Championships.^[3]

2023–24 season

Marsak began the season with a Junior Grand Prix appearance, coming fourteenth at the 2023 JGP Turkey. He then competed on the Challenger circuit at the 2023 CS Finlandia Trophy, where he finished fifteenth.^[3]