Novak Djokovic's dead giveaway for his tennis future became clear at the U.S. Open

The first round of the U.S. Open provided one of the most stunning upsets in recent memory. The legendary Novak Djokovic couldn't get rid of 25-year-old Argentinian Mariano Navone , who outlasted Djokovic and came away victorious.

The caveat, of course, is that Djokovic was not physically well . He was spotted vomiting multiple times throughout the match , which undoubtedly affected his play. But aside from his body betraying him on that specific day, the end of the match may have proved more telling than anything else.

Djokovic simply couldn't hold back the tears after Navone notched the match-winning point. For a player who is always so even-keeled and serious, seeing Djokovic cry had many wondering if this is truly the beginning of the end for a true icon of the sport .

Novak Djokovic may have to ponder retirement due to his health

As it turns out, his health may actually be the reason those tears came out . Djokovic admitted after the loss that Father Time could very well be finally catching up to him.

I didn’t enjoy myself. I think it was obvious that it was an awful feeling on the court for me. It’s something that I’ve been carrying, unfortunately, pretty much every match this year: vomiting, throwing up, a serious issue there. Then my whole body starts to collapse, basically cramping and pain. Yeah, the back just gave up in the end and shoulder.

Djokovic had revealed in early August that an underlying health issue rears its ugly head particularly in hot and humid environments . Of course, those types of conditions are common during the tennis season. Djokovic will seriously have to consider if it's time to hang it all up sooner than later, for the sake of his health.