Bernard Lagat of the US (L) races through a corner in his men's 5,000 metres heat at the IAAF World Championships in Daegu Sept 1, 2011. [Photo/Agencies]

DAEGU, South Korea - American Bernard Lagat ran like "a million bucks" to win his 5,000 metres heat at the world championships on Thursday, while Britain's Mo Farah managed to stay out of trouble in a physical second heat to cruise into the final.

Berlin champion and world record holder Kenenisa Bekele, who returned from a two-year injury lay-off only to limp out of the 10,000 on Sunday, had decided not to run and will instead focus on his preparations for the London Olympics.

Ethiopian Imane Merga flashed a wide grin at Farah as he sped past him to win the second heat in 13:37.96, and received a pat on the back from the Briton for his efforts.

Merga had taken bronze in the 10,000 final, where favourite Farah finished second.

At 36, Kenya-born Lagat is the elder statesman in the field but his legs looked full of running as he recorded the quickest time of the morning heats in 13:33.90.

Lagat, who won gold in the 1,500 and 5,000 at the 2007 world championships in Osaka and has been a force in distance running for a decade, shaved off the beard he had in the lead-up to Daegu and said he meant business.  

"Now I'm serious," he said, having received a pep talk from his son Miika before his heat. "He asked me how I felt and I said, 'like a million bucks.'

"He told me: 'Then go out and run like a million bucks.'"

Lagat had said before the championships in South Korea that he would not run the 1,500 and would focus on the longer distance.

"I won (the heat) and it is a great motivation for the final," he added. "There are a lot of strong guys here. I want to be ready mentally and physically to compete against the best 5000m runners."

Also among the qualifiers for Sunday's final were American Galen Rupp, Ireland's Alistair Cragg and Kenyan Isiah Koech.

The crowd reserved a special round of applause for Gabon's Christian Ngingba, who finished more than five minutes behind Merga in the second heat to record his personal best.