Willis was trapped among the back-markers and in no position to attack over the last 400m.
He was nevertheless content with his effort after struggling to overcome shin splint problems that affected him early in his competitive season.
"I'm surprisingly pleased and happy. Eighth in the world off a ten weeks training, I can't really complain with that," he said.
"Those guys beat me by six seconds in Monaco, three seconds today so the improvements were happening but obviously not in time."
Willis, 34, has never won a world championships medal in six attempts.
He admits it was a stiff challenge to improve on his best result, a sixth placing in Beijing two years ago.
Willis said even if he had been closer to the leaders on Monday morning (NZT), he would have lacked his usual finishing kick.
"Even if I'm up there and got no finish, I've got to try and stay in contention and have something left for the last 200m. But the other guys were much too good today."
Earlier, Quentin Rew secured a place in the New Zealand record books with an expertly-timed performance in the 50km walk.
Rio Olympian Rew surged home 12th, clocking a national record three hours 46 minutes 29 seconds.
It eclipsed Craig Barrett's 16-year-old previous national mark by 1min 34sec.
Rew was 29th after the first 5km but spent the rest of the race making up places, improving six spots over the last 5km in hot conditions.
"The race for me was just putting myself in a good position so that when people did come back I could take advantage of that and go past them," he said.