Frenchwoman Parry, 23, earlier upset American fifth seed Ann Li 7-6(7), 6-4 in two hours and three minutes in the first semi-final to book her place in the final.
In the second semi-final, Bartunkova made a blistering start, breaking Mertens in the opening game before racing to a 4-1 lead. The 20-year-old secured a second break and never allowed Mertens a route back, closing out the set 6-1 after breaking serve for a third time in the final game.
After being outplayed in the opening set, Mertens battled back in the second from a 3-0 deficit. The 30-year-old turned the tide by breaking in three consecutive return games and winning six of the final seven games to take the set 6-4 and draw level.
Mertens carried that momentum into the decider, securing an early break for a 2-1 lead. She tightened her grip on the contest with another break in a marathon seventh game before serving out the set 6-2 to complete the comeback victory.
Follow the Monterrey final with Flashscore!
The US Open women's singles main draw begins on Sunday.
US Open 2026
The US Open is the fourth and final Grand Slam of the tennis season, running from August 30th to September 13th. It is a hard-court tournament held at Flushing Meadows in New York City. Follow the links below for more information!
All you need to know | Men's qualifying | Women's qualifying | Latest US Open news