 Besides his successes at Wimbledon, he won the Boys' singles at the French Open, and reached senior quarterfinals in the US Open in 1953.With Althea Gibson, Nielsen won the US Open mixed doubles in 1957, thereby becoming the first Dane to have ever won a Grand Slam event as a senior.During his long career he won around 30 international titles, played 96 Davis Cup matches for Denmark (with a 53–43 record), and holds the record of having won the most Danish national tennis championships (50).