Casper started playing tennis at the age of four with his father, a former Top-50 tennis player. At the age of 15, he made it to the quarterfinals at the US Open Junior Championships and started the 2016 season ranked No. 1 for singles in the junior tour, becoming the first Norwegian to do so.

After turning pro in 2016, he played his first ATP Challenger at the Copa Sevilla and won the tournament. He broke into the Top-50 in January 2020 after defeating Fabio Fognini and John Isner at the ATP Cup. He won his first ATP title at the 2020 Argentina Open and has since won four more titles, which earned him a career-high ranking of 12.

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