By Justin Palmer

LONDON (Reuters) - Olympic 10,000 meters champion Mo Farah again rode a wave of fervent home support, cranking his aching legs back into life to reach the 5,000 final as decathlon world record holder Ashton Eaton began his gold bid in impressive style on Wednesday.

Farah capped a golden night of track and field for Britain on Saturday with a memorable victory and was afforded a huge ovation on his return to the Olympic stadium, again packed to the rafters for the morning session.

The Somali-born runner is bidding to join an illustrious group to secure the long-distance double at a Games, including Emil Zatopek (1952), Lasse Viren (1972 and 1976), Miruts Yifter (1980) and Kenenisa Bekele (2008).

Farah qualified third from the first of two heats but it was far from plain sailing for the 29-year-old.

"I just kept tripping up, they kept catching me. It was pretty difficult. The 10k took more out of me than I thought," Farah said.

"There was lots of pushing and shoving, but that's what happens in a heat. The final will settle down."

Lining up alongside Farah in Saturday's final will be former child refugee and South Sudanese-born Lopez Lomong.

The 27-year-old Lomong, one of the "Lost Boys" from war-ravaged Sudan who found a home in the United States and was the U.S. flag-bearer at the Beijing Olympics, reached his first Olympic final having stepped up from 1,500 meters this season.

American Eaton came into these Games as one of the hottest favorites for gold having broken Roman Sebrle's 11-year decathlon world record at the U.S. trials in June, setting a new mark of 9,039 points.

Eaton, 24, the 2011 world silver medalist, holds a 105-point lead over compatriot and world champion Trey Hardee after the opening three events, helped by a lifetime best throw of 15.48 meters in the shot.

"I feel good and my body feel good," he said. "It was a fantastic morning."

There was bitter disappointment for 37-year-old Czech Sebrle, the 2004 Olympic champion, whose participation in his fourth and final Games lasted just one event.

HEEL INJURY

Sebrle finished last in his 100m heat and pulled out of the long jump, unable to continue after failing to shake off a heel injury.

"My image of saying goodbye to the Olympic Games was not this one," he said.

"I expected it to be better. It is the Olympics. I kept hoping until this morning."

There was notable interest in the women's 800 meter heats where Sarah Attar became the first female track and field athlete to represent Saudi Arabia at an Olympics.

The 19-year-old, who wore a white head cover, a long-sleeved green top and black leggings and sported luminous green running spikes, received a generous ovation as she trailed in last of the eight runners.

Attar, who clocked two minutes 44.95 seconds - over 43 seconds behind heat winner Janeth Jepkosgei Busienei of Kenya - is the second Saudi woman to compete at the Games following judoka Wojdan Ali Seraj Abdulrahim Shaherkani.

The International Olympic Committee (IOC) had extended a special invitation to Shaherkani and Attar after it pressed Saudi Arabia to end its ban on female participation.

"It is the hugest honor to be here to represent the women of Saudi Arabia," said Attar, who has dual U.S. citizenship.

"It is an historic moment. I hope it will make a difference. It is a huge step forward. It's a really incredible experience."

Defending champion Pamela Jelimo of Kenya and South Africa's former world champion Caster Semenya eased into Thursday's semi-finals.

Like Attar, Turkey's Merve Aydin also received an ovation from a British sporting crowd who love gallant triers when, suffering from an apparent injury, she slowed to a near walk in the home straight but still crossed the finish line.

There were no dramas, however, for Australia's defending Olympic pole vault champion Steve Hooker whose season has been blighted by a crisis of confidence.

Hooker made no mistake with his morning's one and only jump, a clearance of 5.50 which was enough to make Friday's final.

The qualifiers did produce one major casualty, Poland's world champion Pawel Wojciechowski bowing out after no-heighting at 5.35.

(Editing by Ed Osmond)