I didn't see the injury happen. I had a work meeting, so I dropped off of the streaming right after Sinner won the first two sets. He was looking fantastic and blowing Khachanov off the court. However, when I tuned back in towards the end of the third set, Sinner was just stiffly arming his serves in, going for a winner on anything close to him, and he was not able to run after anything that was in the corners. It looked like something was wrong with his lower back or hip area. He looked sick to his stomach because he knew this match was his to win. Work got in the way for me again, so I missed most of the rest of the match, but I was following the score on my phone. He dropped the 4th set 6-0 and was down several times on his serve in the 5th set, but somehow got it into a tiebreak. I tuned back in to watch the tiebreak and he did look dejected in the end as this would have been a great win. Hopefully, he can learn from this, get healthy, and get a good draw at the French Open to make a run. He's going to be a beast when he's fully matured.
Here's how the US Open site reported it:
"No. 11 seed Karen Khachanov of Russia needed five hard-hitting sets to outduel Jannik Sinner of Italy, 3-6, 6-7, 6-2, 6-0, 7-6, on Day 2 of the 2020 US Open.
Khachanov entered today’s contest against Sinner as one of 14 seeded men in the tournament under the age of 25. Next time at the US Open, there is little doubt that Sinner, who turned 19 two weeks ago, will be part of that promising group of young players. He began 2020 as the youngest player in the ATP’s Top 100 and showed in this first-round match against Khachanov why he is one of the fastest-rising players on the men’s tour.
The two players engaged in a tremendous slugfest in the first two sets, with both players rocketing forehands and backhands with abandon. Sinner took both sets by being just as potent as Khachanov but a bit more precise, hitting more winners and making fewer errors.
The 6-foot-6 Khanchanov remained undaunted, however. When Sinner’s level play dipped early in the third set—as he began to show signs of physical discomfort and eventually needed to call for a trainer—Khachanov was ready to take charge. With Sinner moving gingerly, Khachanov won the third and fourth sets comfortably, and seemed ready to close out the match quickly in the fifth set.
Yet Sinner refused to give in. Although moving slowly after every point, he managed to hold serve, game after game, through the fifth set to force a decisive tiebreak. Khachanov, to his credit, never yielded either. He jumped out to the early lead in the tiebreak, and when Sinner hit a backhand into the net at the three-hour-and 44-minute mark, Khachanov’s two-sets-from-behind comeback was complete."