He knew Willis, an Olympic medal winner, would be fast so if he could stay close to him, he knew he'd be up for a personal best (PB).
"I'd kind of been in shape for it for a while so that was the goal. I was like ... 'I'm confident I can run faster than this record'. It was just a matter of when I was going to get it," Tanner said.
"It was a bit windy, a bit gusty on the back straight but I was pretty happy because I got to tuck behind Nick Willis and Julian Oakley for a lot of the race.
"I knew I was on track for a fast time at 400 to go and I was like 'sweet, I just have to hold form and relax' and see what happens at the end."
At the end of that 1500m, there was no big celebration because no one really knew about the record being broken, but Tanner was happy.
"I was just stoked, I was like sweet, I've got a NZ record from a NZ record holder.
"It was just myself and Nick Willis and my crew who knew. It was just a good race, a good day to run fast times.
"Nick Willis was pretty stoked. It was pretty cool because he's one of my heroes and for him to kind of pull me through ... I was pretty appreciative of it.
A few days later at the Capital Classic Meeting in Wellington, Tanner came up against Willis again but this time, the Tauranga teenager was able to take the win.
According to Athletics New Zealand, Sam Tanner outkicked Nick Willis to win the 800m in a PB 1:49.42. Willis was second in 1:50.05 and Sam Petty third in 1:51.54.
"I ran a time of 1:49.42 and that was a big two second PB for me. I knew going into that race I had the potential to break 1:50 and that was kind of the goal and to get 1:49.42, I was like 'sweet i cannot complain about that, and to get the opportunity to give Nick Willis a crack in the last final 50m was pretty cool too.
"I just put my head down and started gassing it and managed to pass him and I was like 'woah what's happening', it was pretty surreal but I was frothing.
"I knew I had the potential to pull one out of the bag but it was a little bit unexpected so I was pretty excited."
What happened at the end of that race is something that will remain etched in Tanner's memory.
"One moment that was kind of super cool for me was when Nick Willis, my hero, started celebrating me and held my hand, a little bit of bromance, and ran down the home straight, or ran back the home straight, celebrating me and my success and that was pretty cool, like a bit of a handover from the expert middle distance runner to the rookie. So that was pretty cool."
Speaking from Michigan in the US, Willis said over the last year he had heard about Tanner's talent and was pleased to have been able to meet him and his family, describing them as "really good people".
He wasn't bitter about his record being broken at all and believed he had held it long enough.
"I was actually really excited.
"He's a nice kid and seems humble about it, that says alot of Sam. He has a lot of things going for him."
Willis said the fact that Tanner was having a lot of success while being relatively new to the sport was exciting because it meant there was still plenty more improvement that could be made.
However, he said it was important not to put a lot of pressure on Tanner and believed the fact he had other interests as a surfer and skateboarder was important for a young athlete's longevity in the sport.
He says Tanner has the endurance and speed and now will need to focus on durability and ensuring his body can last a long-term career at elite level.
This kind of training will no doubt be what Tanner can expect when he heads to America later this year, to start a 4-5 year track and field and cross-country scholarship at the University of Washington, where he will study engineering.
A couple of weeks ago Willis was approached by Andy Powell, head coach of Track and Field and Cross-country at the University of Washington, wanting some intel about New Zealand's operations and how to help Tanner.
It is because of this Willis, who will return to New Zealand next summer, believes Tanner has made the right decision on where to attend.
"I was really encouraged. Sam's going to a place where they've got his best intentions."
Tanner can't wait and is looking forward to what a change in lifestyle going to America will bring him.
"I've had contact with them [the university] for a while but I went on visits in October 2018, and then made the decision pretty close to that. End of October, maybe the beginning of November I kind of knew where I wanted to go and then the details and stuff was kind of finalised in December-ish.
"Because of the scholarship I get a free education so that's pretty exciting, a free degree's a good degree, right?"
He's also excited to get the chance to race more experienced and higher profile athletes, to be pushed and work towards heading to Olympics in the future.
"Those are my main goals anyway."
There is no doubt Tanner has had a great start to the year but he has plenty to do in between now and his departure.
"For the rest of the year I'm just trying to sit down and regroup with the coach and then see what the plan is before I go to the University of Washington."
Tanner's coach of about two years Craig Kirkwood described the middle distance runner as a great, natural athlete who "has done some amazing things".
"He's a great athlete outside of running, great speed and athleticism.
"I always knew he was an extremely talented kid even before I coached him."
Kirkwood, who has been involved in endurance sport since he was 14 and competed on the world stage many times, says when Tanner approached him to be his coach there was never any doubt he would take on the role. He had wanted to coach him for some time.
In the lead up to Tanner's departure, he will compete in the New Zealand Track and Field Championships in Christchurch in March and potentially the Australian nationals, too, focusing on both the 800m and 1500m races.
Kirkwood says they haven't decided what will follow but there are plenty of national and international opportunities for Tanner before he heads to America.
He says Tanner has a good chance of making the Olympic team in 2024 and while 2020 isn't far away, he wouldn't be surprised if he reached that level in time for it.
