London arrived in numbers to celebrate another year of Roger Federer, but his fans left disappointed as Kei Nishikori became the first man to beat him in straight sets in the round robin stages of the ATP Finals. After a tumultuous, but error-strewn night, David Law was joined on The Tennis Podcast by Grad Matt Roberts to discuss what happened. What happened to Federer? Why did he lose his temper with umpire Damien Steiner? And what did Nishikori do differently to beat a man he had trailed 2-7 in head-to-head meetings?Earlier, David and Matt were joined by Simon Briggs of the Telegraph to cover the afternoon’s match between Kevin Anderson and Dominic Thiem. Why does Anderson always beat Thiem, and how much further can he go?Memories of O2 events gone by are discussed, including the Federer-Wawrinka-Mirka saga, and Andy Murray reaching World No.1 in 2016. In the Fed Cup final, the Czech Republic defeated the United States 3-0, but the third rubber was an absolute epic. Why are the Czech’s such an amazing tennis nation?In coaching news, World No.1 Simona Halep and Darren Cahill parted ways. Find out what the podcast team make of it all. The Tennis Podcast is produced weekly in association with Telegraph Sport, and daily at the Grand Slam tournaments.  See acast.com/privacy for privacy and opt-out information.