Sprint king Bolt, who regained his 100 metres title on Sunday, appeared with strapping on his right foot and told reporters after jogging through his morning 200m heat that he had suffered a mishap in training.
"I accidentally dropped a starting block on it," the Jamaican, chasing a third successive world title at the sprint distance he prefers, said after barely breaking sweat to ease into the semi-finals later in the day.
"My foot is feeling better. It was sore. I've worked on it for the last four days."
While Bolt admitted he was "not a morning person", Australia's eager Sally Pearson began the defence of her 100m hurdles crown with a season's best time.
Isinbayeva has hogged the headlines since winning a third pole vault world title on Tuesday in front of an ecstatic home crowd.
The 31-year-old basked in the adulation but found herself facing a barrage of negative headlines when she criticised a gesture of support for the Russian gay community by Swedish high-jumper Emma Green-Tregaro.
STANDARD PEOPLE
Isinbayeva told a news conference in English on Thursday that Russians considered themselves "normal, standard people."
"We just live boys with women, girls with boys," she said,
Green-Tregaro competed in qualifying with her fingernails painted in the colours of the rainbow flag used by the gay movement.
Isinbayeva's comments were condemned as "behind the times" by American 800 metres silver medallist Nick Symmonds and the Russian, in a statement released by the International Association of Athletics Federations, said she respected the views of fellow athletes.
"English is not my first language and I think I may have been misunderstood when I spoke yesterday," Isinbayeva's statement said.
"What I wanted to say was that people should respect the laws of other countries particularly when they are guests.
"But let me make it clear I respect the views of my fellow athletes and let me state in the strongest terms that I am opposed to any discrimination against gay people on the grounds of their sexuality."
HOT POTATO
The controversial Russian law has become a political hot potato ahead of next year's Sochi Olympics, when it will apply to athletes and spectators.
The anti-gay law, which was passed in June, has threatened to overshadow the world championships where the sport's top names have competed in a Luzhniki stadium that has not been full for a single session.
Bolt, on autopilot, went through the motions on a bright and sunny morning as he clocked a pedestrian 20.66 seconds.
The Jamaican still hoped, he said, "to do something special" when it came to showtime.
"Anything is possible once I'm in the final," said the world record holder who, with compatriot Yohan Blake not competing in Moscow, lacks a serious rival to his sprint dominance.
While Bolt cruised, Pearson let a "caged tiger" out on to the track, the Australian putting days of frustration of waiting to appear behind her with a season's best effort.
Pearson, the finest female hurdler since the versatile Gail Devers won three world titles in the 1990s, brushed off the cobwebs to register 12.62, but the 26-year-old said she had plenty to work on.
CAGED TIGER
"It was good to finally be out there. I feel like a caged tiger again. I hate being at the end," she said.
"The start was good... (I) have a bit of tidying up to do at the end of the race. I thought I had a faster time but I screwed up the last hurdle and a half."
Pearson will have to up her game in the semi-finals and final on Saturday, though, with young American Brianna Rollins a potent threat for gold.
Rollins, 21, ran the fastest time in 21 years when she clocked 12.26 seconds to win the U.S. trials in June and her 12.55 was the best of the morning heats.
Defending triple jump champion Christian Taylor progressed safely through to Sunday's final with the third best leap in qualifying, five centimetres shy of Frenchman Teddy Tamgho.
Russia's defending champion Maria Abakumova topped the list in javelin qualifying but there was disappointment for Jamaica's women's 4x400 relay team who were disqualified and will miss the final following a lane violation.
There should be plenty of track excitement later on Friday when American Allyson Felix goes for a fourth world 200 metres title and Briton Mo Farah seeks to add 5,000 metres gold to his 10,000 title.
(Editing by Ken Ferris)
