The distinctively tall figure of Bolt, dressed in a black, yellow and green Jamaican t-shirt, strode around the sunbaked training track in Moscow in front of a crowd of fans and journalists.
As some of his team mates came over to talk to the media, the triple Olympic champion in 2008 and 2012 put on headphones and walked across the infield in the opposite direction, pausing only to greet other athletes on his way.
The world 100m and 200m record holder will start his bid to regain his world 100m title - which he lost to compatriot Yohan Blake in Daegu after false-starting in the 2011 final - in Saturday's heats. If there are no hiccups, he will line up in Sunday's final as hot favourite for gold.
Bolt trained with a large plaster taped behind his right calf but Mills said it was to protect a small cut.
"He's in good health," Mills told Reuters as pop music drifted on the breeze from the sports complex's building. "It's a scrape from a spike, it's nothing."
The atmosphere was relaxed as athletes from various nations jogged around and took massages on tables dotted around the track, but Mills said Bolt was fully focused on maintaining his sprint dominance.
"He's always highly motivated going into the games. He takes competition and the big occasions very seriously. He's highly motivated for games like the worlds and Olympics.
"His training the last five weeks has been fairly good. It wasn't that good going into the Jamaican trials (in June). But he's in good shape now to run very well."
Bolt, dogged by a hamstring injury in the early part of his season, admitted he was still "race-rusty" when he warmed up for the worlds with a comfortable 9.85-second win at the London Diamond League at the end of July, despite a dire start.
Mills said that if the blue Mondo track in the Luzhniki stadium, situated by the banks of the Moskva river, was fast, then Bolt would run fast.
"The track in Berlin (at the world championships in 2009 when Bolt set his world records) was extremely fast. If he gets a fast track he's going to run fast."