He was born in Copenhagen, and was the first Danish tennis player ever to have played in a men's singles final in a Grand Slam tournament.

Besides his successes at Wimbledon, he won the boys' singles at the French Open and reached the quarterfinals in the U.S. Championships in 1953.

[4] After ending his active career, Nielsen held numerous honourable positions in leading international tennis associations as well as served as the supervisor and referee at many Grand Slam events.

[6] His grandson, at the 2012 Wimbledon men's doubles event, became the second Dane to win a Grand Slam tournament.

The following lists main draw appearances for Grand Slam and pre-Open Era Professional Major tournaments only.