Nothing from memory stands out as the singlest highest. Some of the highest, just running a casual search: Navratilova was at 67% late in the '87 W final vs. Graf. She had 69% in the 1990 W final, that's probably one of her highest. Cash had 71% late in his win over Connors, 87W, and he had 60% in the final. He was at 63% in the 84 USO semis and 88 AO final, late in those matches. Edberg at 1992 USO - 70% vs. Lendl, 58% vs. Chang, late in those matches. Mac is tough, he played before net stats were commonly given on TV and in the media. He had 67% late in the '84 USO final, that's the highest I can recall for him.