Willis' result comes two weeks before the Olympic 1500m heats.
Later at dinner, The Willises were seated at the Prince's table, and talked with him about running - Prince Albert also ran the 1500m at university in the US - as well as their shared loves of deep sea fishing and the All Blacks' Rugby World Cup victory over France.
Now Nick, his wife and family are heading off to Italy for a few days' break - before knuckling down for the run home to the Olympics.
"I'm going in with the fifth fastest time in the world, and I'm by no means the favourite. But I would be foolish not to give it a very serious go.
"And many pundits would back me based on my ability to raise my game in a championship setting when there's no pacemakers."
"My target is the gold medal."
Soon after smashing his personal best, the ecstatic 29-year-old Tweeted: "There is great joy to be had when you persevere through seemingly endless injuries, only to come out stronger on the other side.
"Nothing beats running a big PB! No amount of money or fame can top knowing you've taken your body beyond where it has been before."
Javelin thrower Stuart Farquhar was the only other New Zealander in action at Monaco, finishing in sixth place with a best throw of 75.84m.