It was no wonder she was nervous - two big names, Greek-Cypriot Marcos Baghdatis and No2 seed two Tommy Haas of Germany, were knocked out of the competition earlier in the day.
Miss McKinley sat front row in Isner's designated players' box with his coach and chiropractor.
It was her first trip to New Zealand and the couple, who are staying at The Heritage, have dined at Simon Gault's Euro and taken a sailboat tour around the harbour.
She tours with Isner when not studying and has become close to many other WAGs, including Andy Roddick's wife, Swimsuit Illustrated model Brooklyn Decker.
South African Kevin Andersen's American wife, Kelsey, also sat courtside as the fourth seed played Steve Johnson.
New Zealand player Artem Sitak's artist wife Anya accompanied him earlier in the week before his loss to America's Donald Young. And doubles player Lukas Dlouhy's partner is expected to support him when he plays in the quarter-finals today.