Complex sentence: The United States Open Tennis Championships is a hardcourt tennis tournament which is the modern version of one of the oldest tennis championships in the world, the U.S. National Championship, for which men's singles was first contested in 1881.
Split into simpler sentences: The United States Open Tennis Championships is a hardcourt tennis tournament. The tournament is the modern version of one of the oldest tennis championships in the world, the U.S. National Championship, for which men's singles was first contested in 1881.