"I Just felt a little over-extended and tried to relax and a bunch of other guys came through. I knew seven 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."
The 1500m runners also had a short recovery from the heats late yesterday, with Willis finally getting to bed in the early hours of the morning.
"It felt like a pretty quick turnaround, doing an evening race, not much sleep after that and bouncing back, but hopefully I'll bounce back pretty good from that one."
Willis has been on a slow burn this season, only qualifying for the championships at the last possible moment, after shin splints disrupted his off-season preparation.
"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. I'll be doing everything I can for a medal."
A world championship medal is really the only distinction missing from Willis' career CV - he has medals at Olympics and world indoor championships, and won the 2006 Commonwealth Games gold.
This is his sixth appearance at the event and his best performance was sixth two years ago.