Was this player a male? Was it prior to the Open Era? Pretty certain that J Mac's record is the best for a male in the Open era. Here are some other dominant seasons by an ATP player:
1974 ... 95.9% ... 93-4 ... Connors
1984 ... 96.5% ... 82-3 ... McEnroe
1986 ... 92.5% ... 74-6 ... Lendl
2004 ... 92.7% ... 76-6 ... Federer
2005 ... 95.3% ... 81-4 ... Federer
2006 ... 94.8% ... 92-5 ... Federer
2011 ... 92.1% ... 70-6 ... Djokovic