Justin Timberlake has a new biggest fan ... a 13-year-old brain surgery survivor ... who's so impressive that JT stopped his entire media tour yesterday to spend some quality time with her.
JT was being rushed through an NYC building on his way into a live radio interview Tuesday, when he crossed paths with Tina DiLascio -- who told Justin her daughter Giana is a "Make-a-Wish" kid who underwent brain surgery last year to fix a serious blockage in her pituitary gland.
Justin was so impressed -- he stopped in his tracks -- and asked if he could meet Giana, who was waiting outside the building with her brother. We're told JT waited patiently as Tina grabbed Giana ... brought her inside ... and introduced the two.
Tina tells TMZ ... Giana and Justin chatted for several minutes. JT asked if she watched the VMAs and she lit up, telling Justin he's "The next Michael Jackson" ... to which JT bowed his head and said, "Thank you."
Giana asked if she could take a pic with Justin, who agreed ... but only under one condition -- "Only if you make it your screen saver!"
Eventually, JT had to leave ... but not before giving Giana a big hug and telling her to "be well."
And in case you're wondering ... the pic IS her screen saver now.