NEW YORK :Novak Djokovic delivered a lesson in Grand Slam tenacity to Learner Tien in the first round of the U.S. Open on Sunday as he battled past the American teenager 6-1 7-6(3) 6-2 and launched his latest quest for a record 25th major title. Competing in his first singles match since reaching the Wimbledon semi-finals last month, and swapping the whites for a sleek all-black outfit, the 38-year-old Serb fought through physical issues to secure his 80th win at Arthur Ashe Stadium. Victory