American Navarro beats Mertens in Hobart for first WTA title
In their first career meeting, the 22-year-old outlasted the Belgian top seed over a gruelling 2 hours 48 minutes to write another chapter in her burgeoning career.
This time last year, Navarro was ranked 149 but has since soared to a career-high 31, having also reached the semi-finals in Auckland this month.
She is 8-1 this season heading into the Australian Open, where she will make her main draw debut against China's Wang Xiyu as the 27th seed.
After taking the first set in just 35 minutes, second seed Navarro dropped the second as Mertens fought back.
The Belgian, who won back-to-back Hobart titles in 2017 and 2018, looked on track to take the third set and secure the championship when she held a 5-4 lead.
But Navarro won the next two games to swing the momentum back in her favour and closed out the match.