Beijing, Aug 24 (IANS): Olympic 800 metres champion David Rudisha of Kenya almost quit athletics two seasons ago after a setback in his rehabilitation of his career threatening knee injury.

Rudisha, 26, who will be making a serious bid to reclaim his title in Beijing on Tuesday, said he is happy to play the underdog as he seeks the title he last won in 2011 in Daegu, South Korea, reports Xinhua.

"I thought maybe I would never compete in such a big competition when I got this injury which was really career-threatening. I'm glad I'm back and being able to race on world stage, and looking forward to the final," Rudisha said on Monday.

Rudisha admitted he had doubts that he would reach this level of competition again after his injury troubles, which started at the New York Diamond back in 2013.

"Now all I care about is running at the final. It will be good because all the three Kenyans are in the final. But the threat from the other athletes is live and we must be at our peak level," he said on his Twitter account.

Commonwealth Games champion Nijel Amos of Botswana, who was second to Rudisha at the London Olympics, should have been the biggest threat. He has beaten Rudisha on five occasions but he failed to make the cut in the semi-finals.

Kenya will have Rudisha, Ferguson Rotich and Alex Kipketer, with Bosnia's Amel Tuka also seeking the medal in the eight-man race. European champion Adam Kszczot of Poland is gunning for a medal too.

However, defending champion Mohammed Aman of Ethiopia was disqualified in the semi-finals and will miss out on the final on Tuesday.

Rudisha is pencilled to finish his season with a run at the Zurich Diamond League in Switzerland later in September.