Bring sugar, lemon zest, and 2 1/4 cups water to a boil in a medium pot, stirring to dissolve sugar. Cook until a syrup forms, about 5 minutes. Reduce to a simmer. add nectarine; cook until syrup turns pale pink, about 3 minutes. Using a slotted spoon, transfer nectarine to a plate to cool. Transfer syrup to an airtight container; refrigerate about 1 hour.
Peel nectarine, and cut it into large chunks; discard pit. Transfer nectarine to a large bowl. Add peaches and lemon juice to bowl. Using your hands or a potato masher, mash to extract as much juice as possible. Lightly press through a sieve into another bowl; discard solids. Stir in cooled syrup. Juice can be refrigerated up to 3 days. Serve over ice with Prosecco or club soda if desired.