The proper Japanese pronunciation is like “niece-sahn”. In Japanese, it’s written in their phonetic alphabet as:「にっさん」

に is pronounced “knee” (like the knights who say “Ni!”)
っ indicates the following consonant sound is doubled to the end of the previous syllable (so “knee” becomes “niece”)
さ is pronounced “sah” as in a shortened “saw”
ん is pronounced “nn”