Complex sentence: He is the tennis coach of the number one German player, Anna-Lena Grönefeld, of who is an outstanding top twenty doubles and singles player, in mixed doubles, womens'doubles and womens' singles.
Split into simpler sentences: He is the tennis coach of the number one German player, Anna-Lena Grönefeld, of who is an outstanding player ranked in the top twenty for both doubles and singles. She plays mixed doubles, womens'doubles and womens' singles.