By Richard Martin BARCELONA (Reuters) - Cristiano Ronaldo said Real Madrid's late escape act against Sporting Lisbon should serve as a warning after the European champions staged a remarkable four-minute turnaround to open their Champions League defense with a 2-1 win on Wednesday. Ronaldo denied his boyhood club a famous win by cancelling out Bruno Cesar's opener with a free-kick in the 89th minute, before substitute Alvaro Morata continued Real's proud tradition of great European comebacks by grabbing a 93rd minute winner. Despite a euphoric atmosphere at the Bernabeu Stadium at full time, Ronaldo issued a warning to his team mates. More...