Kyrgios was due to face Taylor Fritz in Friday night's match but pulled out shortly before, sending his American opponent through to the semi-finals to face Canadian Denis Shapovalov.
The Australian has had a terrific run of form in the second half of 2022 as he won the Citi Open in Washington and reached his first major final at Wimbledon and the US Open quarter-finals.
"I've been playing amazing tennis all year and actually was dealing with a bit of a knee issue around the US Open time," world number 20 Kyrgios told reporters.
"I got back home and probably didn't take enough time off, to be honest. I went straight back into training. When your body lets you down, it's not a good feeling.
"I know in this case it's probably overloading rather than not being fit enough. I think it's almost being too excited to get on the court and maybe training a little bit too much. So it's positive, but heartbreaking at the same time."
Kyrgios plans to return to the ATP 500 event in Basel and the Masters 1000 tournament in Paris.
