not sure what makes you think that.
it is a known fact that tennis has been declining for a long time, even federer's domination years back in 2004-2006 were weak in terms of field-quality. however, since then it has only gotten worse until the point in which tennis is today. a game of endurance where talent isn't required. furthermore, with an old federer still going strong and still being the best player in the world with young versions of djok-nadal-murray (not to mention a bunch of other players who were never good enough to consistently challenge the top 3-4) you've gotta conclude this is a weak era.
Who says "tennis has been declining for a long time"? Sponsorship is up, prize money is higher than it has ever been. tournaments are recording record-breaking attendances, the sport is being promoted in countries that previously showed no interest in it in the past and the top players are among the richest and most highly regarded stars in sport. The only real decline has been in the US but that is down to the lack of home-grown talent and other markets are happily filling that void.