World Athletics 2013: Dai Greene & Rhys Williams eye 'open' chance
Defending champion Dai Greene believes he can still make his mark in the 400m hurdles at the World Championships despite
Greene's build-up has been hampered by injury and illness, but he won gold in similar circumstances at the 2010 Commonwealth Games.
"Anything is possible in athletics," said 27-year-old Greene.
Fellow Welshman Rhys Williams also feels the event is "wide open" as they prepared for Tuesday's semi-finals.
Swansea Harrier Greene finished fifth in 49.79 seconds in his heat, while Williams came fourth in his heat in 49.85.
Greene's time left him relying on a fastest-loser place, but he was moved up to fourth after Puerto Rico's Eric Alejandro was disqualified.
"It was very difficult out there," said Greene. "I haven't been able to train since the UK trials, really.
"I jumped a handful of hurdles on Friday and fell ill a bit on Saturday as well, so I've had a rough last few days and last few weeks - it's not the ideal prep.
"So I didn't really know I could do this today [Monday]. It was going into the unknown. I'm just happy I got through.
"Hopefully there'll be a bit there tomorrow [in the semi-finals on Tuesday] in the tank."
And Greene insists the Achilles tendon problem that surfaced ahead of the Championships did not lead to thoughts of not defending his title.
"You've just got to try and fight and put yourself in there," said Greene.
"I don't know what I can do tomorrow. It will be interesting to see.
"I had a similar build-up to the Commonwealths in 2010. I was really struggling after the heat and won the final the day after so anything is possible in athletics and I have to look at it this way and try to do the best I can.
"I always believed I was going to be up against it with what's happened.
"But at the same time you don't know how everyone else has prepared for their races so you just hope that maybe they're struggling or they've had one or two knocks through injuries.
Williams admitted he ran "dreadfully" in the heats and the 29-year-old Cardiff clubman said: "Hopefully the cobwebs have been got rid of.
"It's a tough field, but it's wide open and I really want to make the most of that opportunity."
Another Briton, Seb Rodgers, qualified by finishing fifth in his heat in 49.66, the same time as the fourth-placed athlete.