Willis said he felt poor the whole way.
"It felt like a sprint, those guys were too good for me, I wasn't even able to see what happened up front I was just trying to hang on, that's what happens when you race the best in the world."
Willis injured a calf muscle at the Shanghai Diamond League meeting in mid-May. He has only had a mile race in Michigan to get race fit since.
"A 3m 56s mile is still a long way from a 3m 35s 1500m. I was hoping to run a lot better based on indicators. I drew a strong heat and if I can't make the top six I've got a lot of work to do to get through to the final.
"There's every chance with a couple of good nights' sleep that I might improve. I'm still going to race in Europe afterwards because I know it is going to come at some stage, I was just hoping it would come today."
Willis' time has also recently been consumed by fatherhood. He and wife Sierra welcomed first son Lachlan to the world on July 4.
Meanwhile Quentin Rew produced a personal best time of 3h 50m 27s to finish 17th in the 50km walk.
He bettered his London Olympics effort (where he finished 30th) by four minutes 36 seconds.