Do you think Roger Federer is better than Rafael Nadal?
Roger Federer has won more Grand Slam tournaments as well as other tournaments, so I would say yes
Do you think either of them enjoy yoga?
Playing on high level for so many years requires good mental ability but there is no evidence they practise yoga
I think they would enjoy yoga on the beach.  I love doing poses next to the waves.  Do you think they would enjoy that?
Professional athletes have their own means of relaxing so it is possible that they love doing yoga on the beach. However, there is no solid evidence they enjoy it
I would be happy to take a class on the beach with both of those people.  
Both Federer and Nadal will be playing Laver Cup so you might have a chance to meet with them and do yoga on the beach
Federer and Nadal both seem like great atheletes. What is the Laver cup?
Laver Cup is international men's tennis tournament played between two teams: Team Europe and Team World