Early years

Sieber became interested in figure skating after attending an ice show with his family. He took up the sport himself in 2015 at a skating club in Thalwil, Switzerland. Sieber transitioned to his current skating club, Eislaufclub Küsnacht, where he trained as a novice single skater until 2019.^[2] In January 2019, Sieber's coach Cornelia Leroy presented both him and his now partner, Gina Zehnder, with the opportunity to compete as a dance team at the 2020 Winter Youth Olympics, a home event for the Swiss teenagers, and together, they chose to switch disciplines with that goal in mind.^[3]

2019–20 season: Debut of Zehnder/Sieber

Zehnder/Sieber made their junior international debut as an ice dance team at the 2019 Ice Star in October, where they finished 23rd. They gained further competitive experience in the discipline at two additional international junior B events over the fall of 2019, the 2019 Pavel Roman Memorial and the 2019 Bosphorus Cup, where they placed 16th and 19th respectively.

In order to achieve their goal of receiving the ice dance host pick spot at the 2020 Winter Youth Olympics in Lausanne, Zehnder/Sieber needed to be the top junior dance team at the 2020 Swiss Figure Skating Championships in December. Despite being newcomers to ice dance with less than a year of training together under their belts, Zehnder/Sieber won the national title by an over nine-point margin over silver medalists Alina Klein / Maxim Kobelt, and were named to the Swiss Youth Olympic team.

At the Youth Olympics in January, then-14-year-old Zehnder had the honor of lighting the cauldron at the event's opening ceremony as the youngest athlete in the Swiss delegation.^[4] During the ice dance competition, Zehnder/Sieber placed 12th in both the rhythm dance and the free dance for a 12th-place finish overall in the individual event. The duo were also included in the Mixed NOC Team Trophy, grouped into Team Motivation with Ukrainian and Italian single skaters Andrey Kokura and Alessia Tornaghi, and Russian pair team Diana Mukhametzianova / Ilya Mironov. Zehnder/Sieber placed eighth in the team ice dance event, and Team Motivation finished fifth of eight overall.

Zehnder/Sieber concluded their debut season at the 2020 World Junior Championships where they placed 29th in the rhythm dance and did not advance to the final segment of competition.

2021–22 season

Travel restrictions and health concerns related to the COVID-19 pandemic prevented Zehnder/Sieber from competing during the 2020–21 season. The team returned to international competition in September 2021 for their ISU Junior Grand Prix debut at the 2021 JGP Russia. In Krasnoyarsk, the team placed 15th in the rhythm dance and 14th in the free dance for 14th-place finish overall. They replicated this placement at their next event in October, the 2021 JGP Austria. Zehnder/Sieber competed at one final event in the fall in the lead up to their national championships, the 2021 Autumn Talents Cup, where they claimed their first international medal, a silver behind Ukrainian team Myroslava Tkachenko / Andrii Kapran.

In November, Zehnder/Sieber defended their junior national title at the 2021 Swiss Figure Skating Championships, winning gold again by an approximately nine-point margin, this time over compatriots Kayleigh Maksymec / Maxmilien Rahier. The team was initially named to the Swiss berth at the 2022 World Junior Championships but were forced to withdraw after Zehnder sustained a knee injury. Zehnder/Sieber ultimately concluded their season at the 2022 Egna Dance Trophy, where they finished 13th.

2022–23 season

Zehnder's knee injury continued to impact the team's ability to train throughout 2022.^[1] They finally returned to the ice in September, opting to withdraw from their Junior Grand Prix assignments due to lack of preparation time. Zehnder/Sieber opened their competitive season at the 2022 Bosphorus Cup where they placed fourth. The team's only other event during the season was the 2022 Swiss Figure Skating Championships where they placed second in the junior dance category behind Milla O'Brien / Laurin Wiederkehr.

2023–24 season: Junior Grand Prix bronze

Fully recovered from injury, Zehnder/Sieber returned to the Junior Grand Prix circuit for their fourth competitive season as a team. At their first assignment, the 2023 JGP Turkey, the team placed fourth in the rhythm dance, but rose to first in the free dance, ultimately placing third overall. Their bronze medal marked the first medal for a Swiss ice dance team at any ISU Grand Prix event, junior or senior.^[1]

At their second JGP assignment, the 2023 JGP Poland, Zehnder/Sieber set new personal bests in both segments of competition, as well as overall, and finished just off the podium in fourth place.

With Zehnder

With Zehnder