Roger Federer heads to New York for a courtside seat to watch the Brooklyn Nets in NBA

Roger Federer traded a tennis ball for a basketball on Sunday night as he headed to Brooklyn to watch the Nets take on the Boston Celtics at Barclays Center.

The tennis great Federer, a 20-time Grand Slam winner throughout an illustrious career, retired in September in what he called “a bittersweet decision” as the demands of the game ultimately took a toll on his body.

But he was making the most of his new free time on Sunday by attending an NBA game.

Federer had a courtside seat to watch the Nets play – they were beaten 103-92 on Sunday

However, the Nets were unable to record a victory in front of their distinguished guest.

The Celtics defeated them 103-92 on their home court, with Jaylen Brown scoring 34 points for the road team and Kevin Durant adding 31 for the Nets.

The Celtics lead the NBA Eastern Conference with a 19-5 record, while the Nets are 13-12 after Sunday’s result against Federer.

The Nets shared photos of the 41-year-old sitting courtside with the caption “greatness in the building.”

Federer’s retirement in September was an important moment in tennis history and he was considered by many to be one of the best, if not the best, to play the game.

“To my tennis family and beyond, of all the gifts that tennis has given me over the years, the greatest, without a doubt, have been the people I have met along the way: my friends, my competitors and above all the fans who bring the sport to life’, said Federer.

‘Today, I want to share some news with all of you. As many of you know, the last three years have presented challenges in the form of injuries and surgeries.

‘I’ve worked hard to get back in top competitive shape. But I also know the capabilities and limits of my body, and his message to me lately has been clear.

Kevin Durant scored 31 points for the Nets, but they lost to the Celtics with Federer watching

Celtics win at Barclays Center gives them their 19th win of the season

‘I have 41 years old. I have played more than 1,500 games in 24 years. Tennis has treated me more generously than I ever dreamed possible, and now I must recognize when it is time to end my competitive career.

The Laver Cup in London next week will be my last ATP event. I will play more tennis in the future of course, but not in Grand Slams or on tour.

‘This is a bittersweet decision, because I will miss everything the tour has given me.

‘But at the same time, there’s a lot to celebrate. I consider myself one of the luckiest people on Earth. They gave me a special talent for playing tennis, and I did it at a level I never imagined, for much longer than I ever thought possible.’