However he quickly faded and his competition caught up.
Willis, who usually cruises through the rounds at major events, had to find another gear and eventually did enough as he qualified for the final as the first of two "fastest losers".
"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 and suddenly, with about 50 metres to go, I could sense a big rush of guys coming."
"I just didn't quite have that extra gear and had to muscle it in."
Willis' fairly relaxed approach was juxtaposed by that of his long-time coach Warhurst, who had a light-hearted response to the race.
"I told him I'm not going to coaching you anymore if you keep racing like this. I don't have any more room for any grey hairs!"
The former University of Michigan men's track and field coach questioned, but understood, his student's approach.
"Well as usual he always makes it interesting, no matter what. Usually he likes to run from the back, but I think he wanted to make sure that he wasn't getting kicked around or anything and just wanted to get in."
"He doesn't run well from the front, so I don't think you'll see that tactic on Sunday [The men's 1500 metre final on Monday morning NZT]."
An extra day's rest may be what Willis needs to help him prepare for another crack at a maiden world champs medal.
There was little over 24 hours between Willis' heats and the semifinals, however he'll get close to 48 hours between the semi and the medal race.
"Hopefully that extra day's rest will help. Hopefully I'll bounce back pretty good for the final," he added.
"The idea is that I'm improving every race because it's early in the season for me, so that's what I'm going to hang on to. I'll be giving everything I can for a medal ... that's why I'm in the final, to give it a shot."
As Willis swiftly trotted through the media mixed zone, Warhurst left reporters with one final thought.
"Anything can happen. I wouldn't put anything past him. He's surprised people before, both good and bad. It's all to do with where his mind set is after this one."
Charlie Bristow is in London thanks to Air New Zealand.