One of the best American tennis players collapsed on the court in the heat at the US Open

Another American male bowed out of the US Open today, this time in scary fashion.

Jack Sock, the hard-hitting 22-year-old who reached the Round of 16 at this year's French Open, collapsed during the middle of the fourth set in his second round match and had to retire.

He was leading two sets to love before suddenly slowing his serve down substantially and being unable to run during the third set, which he lost 6-4. At 1-2 down in the fourth set, he started to cramp and then collapsed. While the trainers were attending to him, he looked to be fading in and out of consciousness. He was so exhausted and physically depleted that he couldn't even shake opponent Ruben Bemelmans' hand.

The moments when Sock folded (no audio). #usopen pic.twitter.com/3H8OtS4sPA
— Ben Rothenberg (@BenRothenberg) September 3, 2015

Sock down. #usopen pic.twitter.com/hcDi2UH2bH
— Ben Rothenberg (@BenRothenberg) September 3, 2015

As the New York Times' Ben Rothenberg noted, it's now the 13th retirement of the US Open and the 11th by a man. The best-of-five matches are physically taxing, especially during the afternoon sessions at the US Open, where there's often no cloud coverage and the courts are extremely hot and humid.

While there is a heat rule for women's matches that allows players to take a break from play in extreme heat, the rule has not been adopted for the men's game. The Australian Open has its own, separate policy for extreme heat, too. 

Sock will undoubtedly been disappointed with the outcome, as he looked to be well on his way into the third round. Nevertheless, it's curious that this happened less than two hours into the match, and raises questions about whether Sock was thoroughly prepared for the conditions.