Donald Young accused Ryan Harrison of racism at the New York Open and the ATP will investigate.
Beginning the year by winning the Australian Open and returning to world number one means Caroline Wozniacki can take a relaxed approach.
Garbine Muguruza took eight match points to complete her victory over Duan Ying-Ying at the Qatar Open.
Donald Young says he was the subject of derogatory remarks made by fellow American tennis player Ryan Harrison, who has denied the claims.
Roger Federer's form late in his career is "amazing to watch", according to John McEnroe.
Ryan Harrison was the only seed to progress at the inaugural New York Open on Monday.
As three wildcards fell in the Argentina Open first round, Thomaz Bellucci and Guillermo Garcia-Lopez progressed.
Third seed Alexander Zverev moved into the second round of the Rotterdam Open on Monday, but his brother Mischa is out.
Monica Niculescu battled back from a set down to send two-time Qatar Open champion Maria Sharapova out in round one.
The Rotterdam Open provides Roger Federer with a chance to write another piece of ATP Tour history by returning to world number one.