Monaco has been a fond venue for Willis. He has bettered his New Zealand 1500m record four times at Stade Louis II, most recently in 2015 with a time of 3m 29.66s to finish fifth.
"It wasn't pretty by any stretch," Willis told NewstalkZB of his latest feat. "But the magic of the Monaco track paid off again, and it was one of the more satisfying 10th places I've had.
"[Monaco's] been a good spot for me and I relied a lot on it, even after things haven't been going as I hoped, that I could pull one out of the bag."
With the world championship 1500m heats scheduled for August 10, Willis wasn't worried about lowering his time as much as building the strength in his legs to handle three rounds of racing.
Hampered by shin splints this year, the 34-year-old began his European campaign late, scheduling just three races before the qualifying deadline.
"Perhaps I need to take a risk in the next couple of weeks and put in some Lydiardesque training. It's a little close to the championships, but that's the type of work I'm lacking.
"I could then do a perfect taper in the final week. We need to make some quick decisions."
Willis said he had been training the whole year, but the injury had been disruptive.
"It was neither here nor there. I'd run three days, then have to take a week off. I maintained my general fitness, but not enough to compete against the best in the world.
"I felt like I 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."
Meanwhile, shot putter Tom Walsh unleashed his biggest throw of the year in Athens, Georgia, close to his American training base of the last three months.
Walsh threw 22.04m as he also seeks to become the first New Zealand male to win a world track and field championship title outdoors. The 25-year-old is the current world indoor champion.
The throw was his best since his national record of 22.21m set in Croatia last year.
"I'm throwing better than I ever have at this time of year, and I'm in the best shape I've been," he said.