Complex sentence: Austin then became the youngest ever US Open champion, aged 16 years and 9 months, defeating Navratilova in the semifinals and winning the championship match against Chris Evert, who was bidding to win the title for the fifth consecutive year.
Split into simpler sentences: Austin then became the youngest ever US Open champion, aged 16 years and 9 months, by defeating Navratilova in the semifinals and Chris Evert in the final. Evert had been attempting to win the title for the fifth consecutive year.