Willis was asked why he seemed to ease up before the finish.
"I just felt a little over-extended and tried to relax and a bunch of other guys came through. I knew seventh would get through from my heat but suddenly, with about 50m to go, I could sense a big rush of guys.
"I just didn't quite have that extra gear and had to muscle it in."
The 1500m runners had a short recovery from the previous day's heats. Willis got to bed early that morning.
"It felt like a pretty quick turnaround [after] doing an evening race with not much sleep afterwards. Hopefully I'll bounce back pretty good from this one."
Across the past generation Willis has rekindled New Zealand's reputation as a middle-distance running nursery. He has 1500m medals from two Olympics, a world indoor championships and triumphed at the 2006 Commonwealth Games, but an outdoor world championship medal still eludes him.
This is his sixth appearance at the event. His best performance was sixth two years ago in Beijing.
Willis only qualified for the championships three weeks ago at the Monaco Diamond League.
"The idea is that I'm improving every race, because it's still early season for me - that's what I'm hanging on to."
Willis' veteran coach Ron Warhurst agreed anything can happen in the final.
"I wouldn't put anything past him, he's surprised people before both good and bad, it's all a matter where his mindset is at for this one.
"As usual, Nick makes it interesting. Usually he likes to run from the back but I think he wanted to make sure he wasn't getting kicked around. He doesn't run well from the front so I don't think you'll see that tactic on Sunday [Monday, New Zealand time]."
"The point of it was to get through. He came into these championships with a big question mark. Everybody thought 'he's out the back door, the old man can't do it'. Well, we got another shot."
Willis will get together with Warhurst to work out a plan for the final.
Heading into the world championships, Willis' priority was building strength in his legs to handle three rounds of racing.
"I could then do a perfect taper in the final week."
He said the shin splints had left him feeling like he had wasted four months.
"I've got no confidence because I haven't got any [significant] training behind me. It's going to be a shock if I get to the podium, but because I've done it before, there's always that sneaky hope."