Hi,
Playing tennis is her favorite sport/s.
I presume sports is the right word in the above sample, but in truth "sport' is used instead. That confuses me a lot. Could you tell me why it's sport instead of sports in the above?
Say 'sport' when you are talking about one specific activity. eg
Tennis is her favourite sport. Baseball and soccer are his favourite sports.
With regard to your example, say it this way. Playing tennis is her favorite activity/pastime. Tennis is her favourite sport.
Clive