The 18-year-old was crying tears of agony rather than joy after demolishing Japan's Misa Eguchi 6-2, 6-2 to move a step closer to a main draw debut at the All England Club.
"It was [the pain] just really bad today," she told Wimbledon.com. "It started as a gradual pain. I couldn’t serve as hard as I wanted to but I thought push yourself for a couple more points and you can be off the court.”
The world No.115, who rose to prominence after beating former 2011 US Open champion Samantha Stosur at the US Open last year ranked just 296, said she picked up the injury two days ago.
Duval isn't one to hang around between points - rejecting the rituals that many players perform - but her progress was clearly hampered against Eguchi.
The teenager polished off the first set with relative ease but lost her opening service game in the second set. Midway through she clutched her back and called for the trainer. Despite immediately breaking back she received a medical time out at the next change of ends.
Much stretching and rubbing ensued and although Duval dropped only one further game on the way to claiming the match she was visibly in pain on the wide backhand stretch, occasionally hobbling between points.
Duval, who resides in Florida and cites grass as her favourite surface, will fight for a main draw place against fellow countrywoman Nicole Gibbs who ousted Germany's Carina Witthoeft 6-4, 6-4.
She’ll prepare for the contest with massage and ice treatment.
“I’m really excited, this is my first professional Wimbledon,” she added. “Playing in the main draw of Wimbledon would be surreal, I look forward to tomorrow and we’ll see.”
Joining Duval and Gibbs in the final round are fellow Americans Madison Brengle, Melanie Oudin, Shelby Rogers, and Irina Falconi.
Elsewhere, No.3 seed Timea Bacsinszky, currently ranked 86 in the world after reaching the last two on the Nottingham lawns, dispatched Hungary’s Melinda Czink 6-3, 7-6 (2). She will meet No.20 seed Veronica Cepede Royg in the final round. Croat Ana Konjuh, who won the Junior Australian and US Open last year, continued her passage through the draw with a 6-7 (6), 6-4, 6-1 win over Germany’s Laura Siegemund and now faces Stephanie Vogt.
However, the remaining British women were less successful in their qualifying campaigns. Promising 16-year-old Gabi Taylor fell to Czech Tereza Smitkova 6-3, 6-4, while Emily Webley-Smith was ousted by No.6 seed Danka Kovinic 6-1, 3-6, 6-1.
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