The actors shot to fame playing lovers in the 1997 blockbuster and were reunited on screen two years ago in the 'Revolutionary Road', directed by Mendes.
But the stress of filming the movie, where Winslet and DiCaprio had to shoot steamy sex scenes in front of Mendes, has been pinpointed as the beginning of the end of their marriage, reported Daily Mail online.
But the British beauty's friendship with DiCaprio, 35, was re-kindled and he became a shoulder to cry on as cracks appeared in her six-year marriage.
"Leo and Kate have been like brother and sister since they starred in Titanic together. And since Kate's marriage started to go south, around the time they were filming Revolutionary Road, they've got closer again," said a source close to the two.
"Kate has always really adored Leo and feels like she can tell him anything. They were so young when Titanic came out, so it's fair to say they basically grew up together, professionally at least. He's been a rock for her," added the source.
Twice-wed Winslet, 34, is currently on holiday in Mexico with her two children.
The couple, who have been married since 2003 and have a six-year-old son, Joe, insisted the break-up was "amicable and by mutual agreement".
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