Olympic 400m champion LaShawn Merritt marked his return to the track after a 21-month doping ban with second place at Friday's Diamond League meeting here and insisted he is a clean athlete. The 25-year-old clocked 44.74 sec with Jamaica's Jermaine Gonzales claiming victory in 44.69 sec, a season best. Chris Brown of the Bahamas was third in 44.79 sec. "I wanted to come out and get a race, knock off the cobwebs. I feel good physically and I now want to get ready for the world championships," said Merritt. "I am clean. My mother and my team believed in me and they will keep believing in me. Now I will go home and work on a few things and get ready for the worlds. "At the worlds, I will be a contender, I always thought that and with a time today of 44.7, I can't complain." Olympic and world champion Merritt tested positive for the anabolic steroid DHEA in three tests between October 2009 and January 2010. Last October an American Arbitration Association (AAA) 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. Merritt, who was not able to take part in the US national championships, was named in the US team last month for the world championships which take place in Daegu, South Korean from August 27. South Africa's 800m world champion Caster Semenya endured a miserable performance, finishing eighth in her event in a time of 2min 01.28sec, with Jamaica's Kenia Sinclair claiming victory in 1min 58.21sec. The 20-year-old, who clocked a jaw-dropping personal best of 1:55.45 in storming to victory at the 2009 Berlin worlds but was then cast into limbo for almost a year because of allegations over her true gender, admitted she had been off colour. "It was one of those days. I am quite happy with my performance even if I didn't run the race that I wanted to. I expected to run faster. I felt a little heavy," said Semenya. "But I keep going. I never quit. Next time I hope to do better." Later Friday Olympic and world sprint champion Usain Bolt takes part in the 200m. But one man missing from the night's programme is Olympic 110m hurdles champion Dayron Robles who pulled out of his eagerly-awaited head-to-head with this year's world leader, the American David Oliver. Robles injured his ankle last week in a false start in Barcelona and with the world championships in mind, he preferred not to risk running in Stockholm. The Cuban is the world record holder (12.87secs) while Oliver has produced the best time this season of 12.91secs.