Katie Holmes has friends in high places -- and that includes Nicole Kidman.

As Holmes, 33, struggled and secretly plotted to end her marriage to Tom Cruise, she found a pillar of support in her husband's second ex-wife, who divorced the actor in 2001.

"They've spoken over the last few weeks," a confidante tells the new Us Weekly of Holmes and Kidman, 45. "Nicole has been supportive, saying she's been through it too and to hang in there." (One way Holmes may have made those secret calls to Kidman? The Los Angeles Times reported Tuesday that the Romantics actress used a disposable cell phone provided by a pal to plot her divorce without the detection of Cruise and his team.)

It's not a brand-new friendship either, the confidante says, telling Us that the women have been speaking since Holmes' 2006 marriage to Cruise, 50. Kidman shares kids Isabella, 19, and Connor, 17, with Cruise, although much has been made of her estrangement from their kids.

"She has been a private friend not many people know about," the source continues.

As Holmes reached the breaking point in her marriage to Cruise -- and with mounting fears about daughter Suri -- she reached out again to Kidman for advice. "Nicole offered her support and help," the insider adds.

Find out much more about Holmes and Kidman's secret friendship and all the details on the biggest divorce of the year in the new issue of Us Weekly, on stands Friday!