The US Open men’s semi-finals did not deliver the competitiveness they promised – partly due to Rafael Nadal’s knee, partly due to Novak Djokovic’s brilliance.On The Tennis Podcast, on his birthday, David is joined separately by Simon Briggs of Telegraph Sport and Catherine to talk about it all.Nadal’s knee meant he had to retire when trailing Juan Martin del Potro by two sets. Is it a coincidence that he’s had to pull out of both hard court slams in 2018? How will he manage his body going forward?On a wider scale, Simon points out that a concerning pattern is developing: a number of showpiece matches this year have been compromised by injury or fatigue. Is tennis becoming too physical? Does something need to change? And is the solution to get rid of five sets?Del Potro’s victory was almost lost in all this, but he played well. His powerful-again backhand, in particular, was impressive. But what chance will he stand against Djokovic in the final?Djokovic beat Kei Nishikori 6-3, 6-4, 6-2. Nishikori actually played pretty well, but Djokovic was simply exceptional. David thinks he is playing as well as ever.Will the crowd influence the final? Del Potro can be inspired by the chanting of his name, but Catherine reckons it will also bring the defiant best out of Djokovic.Finally, who are Catherine and David predicting to win the women’s final between Naomi Osaka and Serena Williams? There will be daily editions of The Tennis Podcast throughout the US Open fortnight. The Tennis Podcast is produced in association with the Telegraph and sponsored by Amazon Prime Video, the new home of the US Open in the UK. See acast.com/privacy for privacy and opt-out information.