Ni won team and mixed doubles gold medals for China in the 1983 World Table Tennis Championships. She moved to Germany in 1989 and settled in Luxembourg two years later. Her husband, Tommy Danielsson, is her coach and training partner. She competed at the 2008 Summer Olympics, reaching the third round of the singles competition. She qualified for competition at the 2012 Summer Olympics in the Women's singles competition. She lost 4–2 to 16-year-old Ariel Hsing from the US in the 2nd round in the London games.

She competed for Luxembourg at the 2016 Summer Olympics in Rio de Janeiro in the women's singles competition. She lost 4–2 to Feng Tianwei of Singapore in the 3rd round. She was the flag bearer for Luxembourg during the closing ceremony.

When she qualified for the 2020 Summer Olympics at the age of 58, Ni became the oldest Olympian table tennis player.

In 2021, Ni, 58 years old, won the women's doubles bronze medal for Luxembourg (LUX) in 2021 World Table Tennis Championships alongside Sarah de Nutte.