By Sudipto Ganguly LONDON (Reuters) – Novak Djokovic stayed serene amid a frenzied atmosphere to tame fiery Australian Nick Kyrgios in an engrossing final on Sunday, lifting a seventh Wimbledon trophy and taking his Grand Slam title count to 21. Rallying from a set down, Djokovic exhibited ice-cool nerves in sweltering conditions to win 4-6 6-3 6-4 7-6(3) for his fourth consecutive trophy at the grasscourt major to close in on Rafa Nadal’s record 22 Grand Slam titles. By lifting the Challenge Cup once again, Djokovic drew level with his childhood idol Pete Sampras in the all-time winners list …