Rafael Nadal Exits 2018 US Open Match vs. Juan Martin Del Potro with Knee Injury

World No. 1 Rafael Nadal bowed out of his U.S. Open semifinal match against Juan Martin del Potro on Friday after his troublesome right knee flared up. 

Nadal was down two sets, 7-6 (3), 6-2 at the time of the retirement. 

As NBC Sports' Nick Zaccardi noted, "Nadal's right leg was taped just below the knee in the first set and again in the second, after he had ripped off the tape. The Spaniard winced and limped in the second set."

"The pain in the knee is always very similar when I have it," Nadal said after the match, according to the New York Times' Christopher Clarey. "This time it was something a little more aggressive because it was in one movement, not progressive."

Prior to the semifinal, Nadal played a grueling schedule at the year's final major. The Spaniard went four sets against Karen Khachanov in the third round and Nikoloz Basilashvili in the fourth round, and he played a near-five-hour thriller against Dominic Thiem in the quarterfinals. 

Nadal was also forced to retire in the fifth set of his Australian Open quarterfinal this year against Marin Cilic because of a leg injury. 

It's unclear if the knee injury will prevent Nadal from hitting the court for Spain's Davis Cup semifinal versus France next weekend.