Daegu (South Korea): Double amputee Oscar Pistorius on Sunday reached the world championships 400m semi-finals as defending champion LaShawn Merritt also qualified - a month after returning from a drugs ban.

The controversial 'Blade Runner', who runs with carbon fibre prosthetic running blades, was third in his heat with a time of 45.39sec, qualifying 14th quickest overall for Monday's semi, with the final being run on Tuesday.

The issue of whether the blades give the South African, cleared three years ago to run against able-bodied athletes, an advantage is back in the spotlight following Pistorius's qualification for the worlds in South Korea.

Pistorius, 24, had both legs amputated below the knee when he was 11 months old because of a congenital condition that meant he was born without fibulae - lower leg bones.

"I am tired. I need to cool down now, to relax, refocus and chat with my coach. I had a good race. I am just pleased to be part of this sport, of this event," said Pistorius, a four-time Paralympic gold medallist.

"Being here has been a goal I have had for many years. I have worked extremely hard to be here and it has been phenomenal to run today. It has been a lot of pressure in the race, and there is a lot of work for tomorrow. I need to be as consistent as today."

Merritt's time of 44.35 was the fastest in the world this year and saw him top the qualification standings.

"I have been training very hard for a while to come out like this today. I will take it round by round. It was a comfortable race. It was not hard for me. Now I am just getting ready for tomorrow. I made my statement today as defending champion," he said.

The American, who also won the 2008 Olympic title, tested positive for the anabolic steroid DHEA in three tests between October 2009 and January 2010.

Last October an American Arbitration Association panel declared that his ban would end on July 27, less than the usual 24-month ban, thereby making Merritt eligible to compete prior to the world championships.

The arbitrators stated they believed his positive test stemmed from an inadvertent action and was not intended to produce a competitive advantage.

Also through to the 400m semi-finals are Rondell Bartholomew and Kirani James from Grenada along with European champion Kevin Borlee of Belgium, whose twin brother Jonathan Borlee also qualified.