As Serena leaves, Nadal loses & Federer absent, is the era over?

Serena Williams played what's expected to be her last match at the U.S. Open. Rafael Nadal lost in the fourth round. Novak Djokovic and Roger Federer weren't even in the tournament. Those four players dominated, and were the main draws, in tennis for decades, collecting a total of 86 Grand Slam singles titles, each with at least 20.