As Serena leaves, Nadal loses, Federer and Djokovic absent, is era over?

Serena Williams, you might have heard, 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 (more on them later) 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. And so, as the quarterfinals began at Flushing Meadows on Tuesday without any member of that quartet present, it made sense to ask: Is this the end of an era?

The 36-year-old Nadal sounded a philosophical note about the topic after he was bounced by 24-year-old American Frances Tiafoe 6-4, 4-6, 6-4, 6-3 on Monday.

"Some depart, others come and the world keeps going. It's a natural cycle," said Nadal, who noted that his wife is pregnant with their first child and so he isn't sure when he will play next.

"It's always the same. The same one have been up there several years; others are coming and we will be leavin