It all happened at the end of the first set in his match against Khachanov. Nadal called for a medical-time out because of a sudden pain in his right knee, and got it taped. He managed to pull out a victory against Khachanov in the end, but his movement was clearly compromised.
Even in the post-match interview, Nadal didn't clarify about the condition of his knee clearly, and left the speculations flow. This can be attributed to the fact that Khachanov, who almost beat Nadal in the US Open 2018, physically wore Nadal down by striking the ball as hard as he did.
Nadal had to put too much much physical effort into winning the match, and that may have taken its toll after the adrenaline had subsided.
Nadal was quoted as saying, "I warmed up today and felt that my knee was not good to compete at the level I needed to compete in the semi-finals. It's tough for me to accept all these things that i am going through in my career."
Now coming to the prospects of the final, Roger Federer would be heavily favored to lift his 101th ATP title and his 6th BNP Paribas Crown. He will face the winner of the match between Milos Raonic and Dominic Thiem, who are currently battling it out on the court.
If Thiem wins, it would be interesting to witness a game of two single-handed backhands in the final. If Raonic wins, he might challenge the Swiss even more with his big serve, which has regained some of its lost venom in 2019.
Tennis fans all round the world would be utterly disappointed with this though, as they have to wait until Madrid in May for Federer and Nadal to have another chance of facing each other.