Complex sentence: He had dominated the event since the start of 2013-he was the only man who had gone beyond the 6-metre mark that year and had cleared the year's highest height of 6.02m just two weeks ago in London.
Split into simpler sentences: He had dominated the event since the start of 2013. So far in 2013 he was the only man to have gone beyond the 6-metre mark and he had cleared the year's highest height of 6.02m just two weeks ago in London.