Olympic champion David Rudisha has pulled out of the 800 metres at next month's world championships with a knee injury, his coach said Tuesday, robbing the year's top track meet of another star attraction after sprinter Tyson Gay withdrew following a failed doping test.

Rudisha's coach, Colm O'Connell, told The Associated Press that the world record-holder had been sidelined from training for six weeks with a right knee injury and had received treatment in Germany, but he wouldn't recover in time to defend his world title in Moscow.

"Finally we have made the decision," O'Connell said in a phone interview. "We held back for long hoping for improvement and he has improved quite a lot but we didn't want to do a crash course between now and the world championships, which may aggravate the injury since he has been out of training."

O'Connell said the 24-year-old Kenyan, who produced one of the Olympics' lasting performances in London last year to win gold in a world-record time, first felt pain in his knee at the Adidas Grand Prix in New York in May. That led to him withdrawing from a meet in Eugene, Ore.

O'Connell said the injury may have occurred a few weeks before New York when Rudisha was running on rough ground in Kenya, where he trains in the high-altitude regions of Eldoret and Iten, but they couldn't be sure. Rudisha sought treatment in Germany and was initially hoping to recover in time for the Aug. 10-18 worlds.

"Doctors there [in Germany] said it may take three weeks for him to recover and we thought he would have seven weeks of training, but it has been longer," O'Connell said. "That's why we have decided to make the announcement."

Rudisha was one of the athletes the IAAF would have desperately wanted to be at the worlds after Gay, a former world champion, said this weekend he would not compete in the Russian capital after he tested positive for a banned substance.

Top Jamaican sprinters Asafa Powell, the former 100-metre world record-holder, and Sherone Simpson, a three-time Olympic medallist, also have failed drug tests just three weeks before the world championships.