Complex sentence: He qualified for the 2011 World Championship in South Korea and reached the semifinal where he ended last timewise, he was 14th in the first round, his personal best at 400m would have given him 5th place in the finals.
Split into simpler sentences: He qualified for the 2011 World Championship in South Korea and reached the semifinal where he ended last timewise. He was 14th in the first round, his personal best at 400m would have given him 5th place in the finals.