Kevin Keegan (the football player and manager) was famous for saying things that were funny because he had used the wrong words or confused two or more expressions.
There are two sayings involved:
"The tide is turning" - the tide of the sea is in the process of changing from in-coming to out-going (or vice versa) -> figuratively, this means the side that was losing is now gaining the advantage.
"The ball is in our court." -> we have the initiative/advantage.
In this case, he was speaking about a game of football in which his team had not been playing well, but he thought that his team had started to improve and now had the advantage over their opponents.