Who is the greatest tennis player of all time?
In the modern era and based on total titles and the overall impact on the game, both Serena Williams and Roger Federer as considered the greatest player of this generation
Instruction: Who is the greatest tennis player of all time?
Context: 
Response: In the modern era and based on total titles and the overall impact on the game, both Serena Williams and Roger Federer as considered the greatest player of this generation