Surgery for which, took Roger Federer out of competitive tennis during 2020 and early 2021?

Question

Where was the record for the longest professional tennis match broken in 2010 when John Isner & Nicolas Mahut took 11 hours and 5 minutes over a three day period?

Answer 1

His knee
Federer returned to compete in May 2021 in the Geneva Open. He went on to compete a month later in the French Open but withdrew after winning in the third round to protect his knee in anticipation of Wimbledon. Federer had had knee surgeries over about four years until early 2020, when he pulled out of competition to recover after the last, the delay coinciding with the start of the COVID-19 pandemic.