Complex sentence: Gilchrist's third ODI century helped Australia to equal the world record for the highest ODI runchase, while his fourth, against Sri Lanka in 1999 helped Australia achieve the highest successful runchase in ODI history at the SCG at the time.
Split into simpler sentences: Gilchrist's third ODI century helped Australia to equal the world record for the highest ODI runchase. His fourth ODI century, against Sri Lanka in 1999 helped Australia achieve the highest successful runchase in ODI history at the SCG at the time.