Complex sentence: Six years after retiring, Jeffries made a comeback on July 4,1910 at Reno, Nevada in a match against champion Jack Johnson, who had won the Heavyweight Championship by defeating Tommy Burns at Rushcutters Bay in Australia in 1908.
Split into simpler sentences: Six years after retiring, Jeffries made a comeback on July 4,1910 at Reno, Nevada. He fought champion Jack Johnson, who had staked his claim to the Heavyweight Championship by defeating Tommy Burns at Rushcutters Bay in Australia in 1908.