Leonardo DiCaprio & Erin Heatherton -- On the Ropes

Leonard DiCaprio's relationship with his 22-year-old Victoria's Secret model girlfriend Erin Heatherton really hung in the balance this week -- as the couple hit up an awesomely awesome zip line together in Mexico ... and we got the pics!

The exact location of Leo and Erin's Mexican vacation is unclear -- but by the looks of the photos, 37-year-old Leo and his little lady friend are having a blast south of the border.

This just in -- Leonardo DiCaprio's life is much better than yours.