Keller began playing goalkeeper after his father bought him goalie gloves as a joke.
[1] He was promoted to their U21s in the 2019–20 season, and was called up to their bench as backup goalkeeper in the Swiss Challenge League.
[2] On 8 January 2020, he signed a professional contract with Grasshoppers until 2020 and was formally promoted to their senior team as third goalkeeper.
[5] He transferred to the Swiss Super League club Young Boys on 12 February 2023 after they activated his release clause after a season-ending injury to their starting goalkeeper David von Ballmoos.
[6] He made his professional debut with Young Boys in a 1–1 Swiss Super League tie with FC Zürich on 14 May 2023.