Complex sentence: Mahut has also become good friends with the American tennis player John Isner, after they played together in the longest professional tennis match ever at Wimbledon 2010, which lasted over 11 hours.
Split into simpler sentences: Mahut has also become good friends with the American tennis player, John Isner. The two became friends after they played together in the longest professional tennis match ever at Wimbledon 2010, which lasted over 11 hours.