The 24-year-old Dibaba, who won the 1,500m gold on Tuesday in dominant fashion, was untroubled in the opening heat of the 5,000 as she won in a time of 15 minutes 20.82 seconds on another hot and sunny morning at the Bird's Nest stadium.
Dibaba crossed the line just ahead of Kenyan Mercy Cherono (15:20.94), while compatriot Almaz Ayana, who won bronze two years ago in Moscow, put down a marker for Sunday's final by recording the fastest time of the day, 15:09.40, in the quicker second heat.
"As everyone knows, Mercy Cherono and Almaz Ayana are challengers. We've been running together many times," Dibaba told reporters.
"Winning the 1500m gold medal gave me confidence, but you never know what is going to happen in the final."
Dibaba showed her pace in the 1500m final by taking the lead with two laps to go and blowing clear with ease. She said a world record was possible in Sunday's 5,000 final.
"If I keep my good performance I can run faster than the current world record," she said.
Asbel Kiprop of Kenya also showed his class in the heats for the men's 1500m, coasting through to Friday's semi-finals in a time of 3:38.97 after winning the second heat.
The defending world champion, who won Olympic gold at the Birds Nest at the 2008 Beijing Games, played down his display.
"My plan was just to be in the top six and to go on to the semi-final," the 26-year-old said.
"It is not about sending a message to anyone. I was relaxed because during my last three weeks of training in Nairobi I was in good shape."
Elsewhere on the sixth morning of the championships, triple defending champion and Olympic gold medallist Reese missed out on a spot in Friday's long jump final after finishing 24th in qualifying with a disappointing 6.39 metres.
It was a better day for Briton Katarina Johnson-Thompson at the pit as she booked a spot in the final with an effort of 6.79m after three long jump fouls cost her a heptathlon medal on Sunday.
Olympic champion Anna Chicherova of Russia was among a group of experienced high jumpers to secure passage through to Saturday's final.
The 33-year-old cleared the required 1.92 metre mark along with 31-year-old double world champion Blanka Vlasic of Croatia and Spain's 36-year-old European champion Ruth Beitia.
Defending champion Brianna Rollins cruised through the 100m hurdles in a time of 12.67 seconds, with American compatriot and 2008 Olympic champion Dawn Harper-Nelson also impressive.
In the men's discus, Fedrick Dacres of Jamaica (65.77 metres) and Pole Piotr Malachowski (65.99) were the only two to pass the 65m automatic qualifying mark ahead of Saturday's 12-man final.
(Writing by Patrick Johnston in Singapore; editing by Amlan Chakraborty)
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