The 34-year-old Farah was roared on by a packed London Stadium on the opening night of action but the victory was far from easy with Farah battling his way through a final lap of bumps and stumbles to win in 26 minutes, 49.51 seconds.
Since winning the 5000m at the 2011 World Championships, Farah has completed the distance double at the 2013 and 2015 Worlds and at the 2012 and 2016 Olympic Games.
This was one of his hardest wins, however. The Briton almost fell in the last lap after making contact with Kenyan Paul Tanui who took bronze behind Ugandan Joshua Cheptegei.
Jamaica’s Usain Bolt also began his campaign to finish his career as the most decorated athlete of all time at a world championships with a frustrating stumble. But the Olympic and world champion still looked comfortable with a first-round 100m heat victory in 10.07 seconds.
“That was very bad, I stumbled coming out of the blocks. I'm not very fond of these blocks,” Bolt said. “I have to get this start together because I can't keep doing this.”
Bolt already has won more world gold medals than any other athlete with 11 titles but he can overtake Merlene Ottey as the winner of most medals with two more in London.
Bolt’s compatriot Julian Forte ran the fastest heat in 9.99 with American Christian Coleman second fastest, winning his heat in 10.01. American Justin Galtin, silver medallist at last year’s Rio Olympics, won his heat in 10.05, the sixth fastest time.
The women’s 1500m is gearing up to be one of the most fascinating races of the championships. Ethiopia’s defending champion Genzebe Dibaba won the first heat in 4:02.67 just ahead of Caster Semenya, the South African Olympic 800m champion who is attempting a middle-distance double in London.
Sifan Hassan of the Netherlands, who has been in strong form this year, won a slower heat in 4:08.89 ahead of Britain’s Laura Muir who finished fourth in 4:08.97.
The biggest shocks of qualifying in the field events were the failures of American world indoor record-holder Jenn Suhr to qualify for Sunday’s women’s pole vault final and of compatriot Jeff Henderson to make the long jump final.
Suhr won a gold medal in the same stadium at the London 2012 Olympics but could not register a mark while Greek favourite Ekaterini Stefanidi needed just one vault to secure her place.
Olympic champion Henderson struggled throughout the qualifying and only managed 7.84m in the final round, back in 17th place.