Defending champion Frances Tiafoe and runner-up Jan-Lennard Struff have been confirmed as entries for the Stuttgart grass court ATP tournament in June, organizers said on Friday. American Tiafoe won his first title on grass at last year’s edition of the event which serves as a Wimbledon tune-up, beating Germany’s Struff 10-8 in a final set tie-break. “I have fond memories of this tournament,” world number 16 Tiafoe said. “Everything was so relaxed, so I’m happy to be coming back to the Boss Open in Stuttgart this year.” Struff will also be present again at the June 10-16 event, after Germany’s…