Hamburg, April 30, 1993. Monica Seles is sitting in her courtside chair, mid-match, breathing hard. Then a man steps out of the crowd and drives a knife into her back. The world watches.

Nobody stops him in time. In that single moment, the most dominant tennis player on the planet is turned from champion into victim — and sport is forced to confront a question it has never properly answered: how safe are its stars? A new BBC Radio 5 Live documentary, presented by Clare Balding, goes back to that day through the voices of people who were actually there. It is not a comfortable listen.

Shock and disbelief, witnesses recall, gave way quickly to something harder — the realisation that crowd security at the sport's elite events had been dangerously, almost casually, ignored. The attacker, a fanatical supporter of Steffi Graf, received a suspended sentence that many observers still regard as a scandal. But the documentary does not begin with the knife. It begins in Yugoslavia, where a teenage Seles is already hitting the ball with a ferocity that coaches find almost unsettling.