Mo Farah began his bid to add world 5,000m gold to his 10,000m silver by qualifying comfortably for Sunday's final.
Farah was second in his heat, passed by Ethiopia's Imane Merga on the line, while Bernard Lagat won the other heat.
"The conditions out here are really hot, everybody has to deal with that. I'm going to get an ice bath now and recover and do what I can," he said.
"My legs are alright - I've had great medical support and great team spirit."
Fellow British medal prospects Jenny Meadows qualified comfortably in the 800m, winning her heat, as did South Africa's Caster Semenya, the defending 800m champion who missed last season following gender testing.
Oscar Pistorius, the first leg amputee to compete at the World Championships, was part of the South African quartet that qualified third in the 4x400m relay for Friday's final in a national record time of two minutes 59.21 seconds, while Great Britain made it as fastest losers.
"I'm more than happy with my outcome and proud to be part of the team," said Pistorius.
"Sometimes the medal does become a bit of a pressure but there are a lot of people that would die to be in the position I'm in."
Farah, comfortably among the five qualification places from his heat, hobbled a little after the race, blaming blisters he gained during Sunday's thrilling 10,000m final, where he was pipped at the line by Ibrahim Jeilan of Ethiopia.
"I'm quite looking forward to the final," said Londoner Farah, who will not have to face Ethiopian double world record holder and defending champion Kenenisa Bekele in the final following his withdrawal with a calf injury on Wednesday.
"I've been relaxing with my wife and daughter. It's is nice to switch off and forget about running. It's going to be good."
Meadows, world bronze medallist two years ago, looked comfortable in qualifying but is still wary at the prospects of Friday's semi-finals.
"There are more than eight great girls in the world this year so I see the semi-final as a little bit more nerve racking than the final," she said.
Marilyn Okoro - who secured her qualification for the Worlds by running an extra race on the eve of the selection announcement - also made it through in the 800, fourth in her heat while Emma Jackson was the fastest loser.
In the javelin, Goldie Sayers threw 62.19 with her second attempt to qualify for the final and afterwards said she believes she is very close to the form that saw her finish fourth at the Beijing Olympics, after missing 2010 to have hip surgery.
"It is boiling out there!" she said. "The finals are all in the evening so its not quite so bad for the heat. I think I've got a chance; there are six of us around the medals so we will just have to see who comes out on top."
Chris Tomlinson scraped in as the 12th and final qualifier for the long jump final, with a leap of 8.02m on a heavily strapped knee, but Greg Rutherford missed out after suffering a hamstring injury.
"I didn't compete very well but I'm in the final, that's the main thing. We all start on zero there," Tomlinson told BBC Radio 5 live.
"It's the World Championship final, there's no way I'm pulling out. If it stands up, good, if not I'll go down trying."
In the women's 200m, Anyika Onuora moved into Thursday afternoon's semi-finals with Veronica Campbell-Brown the fastest qualifier from the five heats.