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.
The Fed Cup played host to the competitive return of Serena Williams on Sunday, as United States saw off Netherlands.
Over a year after her last competitive appearance, Serena Williams made her return but suffered defeat alongside sister Venus.
In an all-Spanish final at the Ecuador Open, world number 107 Roberto Carballes Baena upset Albert Ramos-Vinolas.
Two-time grand slam champion Garbine Muguruza is thrilled to have the benefit of the 'Conchita mind' on her team in Doha.