Willis has also won three Commonwealth Games medals, including gold at the 2006 event.
But still there's a desire to stand atop the podium at track and field's pinnacle competition.
"I was talking about that with my wife, Sierra, the other day. It's almost nice I haven't yet in many ways because it keeps you hungry," said Willis.
"If it's the same old thing every year, and I think maybe [Kenyan middle-distance star Asbel] Kiprop is going through a little of that at the moment, it's hard to stay hungry if you're always at the top, and that's something I haven't really had success at in the past. I've failed quite miserably a few times at the world champs."
Being able to see that as a positive, is testament to Willis' character. A drive that has helped him overcome chronic shin splints which nagged him for much of the Northern Hemisphere spring. He's now fully recovered - just in time to head off to Europe to hit the 1500m qualifying mark for the world champs. The 34-year-old took a break following last year's Olympics, coming home to New Zealand before returning to Michigan to begin training again. The down-time almost backfired, though.
"April, May, June, I was half-training, half-cross training, but then I realised I wasn't getting anywhere with that. So I scrapped everything and took 10 days off and my body suddenly got healthy from that and I've since taken a slow and steady approach and started to regather my fitness over the past couple of weeks," he said.
Willis will compete in Brussels next weekend, hoping to dip under the qualifying time of 3m 37.5s for London (his personal best is 3m 29s).
It's one of three events he has given himself to go under that time, with the window closing four weeks before the start of the event.
He's confident he may just need one take.
"My training is going excellent and everything seems to be tracking as in the past," Willis said.
"That would indicate that I am heading for a really good peak come world champs, so naturally I'm really excited about that." His own aspirations aren't the only thing on his mind, as at the beginning of the year, he teamed up with compatriot and 1500m rival Hamish Carson.
He enjoys coaching the 28-year-old, who's in the middle of his European season and who on Thursday narrowly missed out on qualifying in Sweden, having come close to the A standard in Spain a couple of weeks ago.
Joining forces with Carson has been a "natural evolution" of their relationship, Willis said.
"We would train together so much whenever I was home, with him being a Wellington boy. We would run together most days for two or three months. And then he would come over and train with us in Michigan for two or three months and so even though Arch Jelley was his coach, Jelley would say, 'well, if you're with Nick, you may as well learn from him and do what he's doing'."
It's all part of a bigger trend for Willis, who gets a kick out of seeing fellow Kiwi athletes achieving. Athletics and sailing led New Zealand's medal haul in Rio, and Willis believes more talent is emerging.
"There's a lot of young blood coming into this New Zealand team that have qualified for the worlds this year, like [100m runner] Joseph Millar and Camille Buscomb, who has qualified in the 5km and 10km races," Willis said.
