The International Olympic Committee is to seek an explanation from Russian member Shamil Tarpischev for his controversial comments about Serena and Venus Williams.

Tarpischev, president of the Russian Tennis Federation and captain of their Davis and Fed Cup teams, has been condemned for calling the siblings "the Williams brothers" on a Russian TV chat show.

Serena, meanwhile, opened the WTA Finals in Singapore with her 16th consecutive victory at the end-of-season tournament following a 6-4, 6-4 win over Ana Ivanovic.