Novak Djokovic had to battle back from two sets down to reach the Wimbledon semi-finals and preserve his remarkable Centre Court record.The defending champion, bidding for a fourth consecutive title, looked in danger of a sensational defeat when Italian youngster Jannik Sinner led 2-0.But for the seventh time in his career, 20-time grand slam winner Djokovic overturned a two-set deficit to win 5-7 2-6 6-3 6-2 6-2.Amazingly, it means Djokovic has still not lost a match on Centre Court in nine years – since Andy Murray beat him in the 2013 final.His only two defeats since, to Sam Querrey in 2016 and a retirement against Tomas Berdych a year later, came on Court One.But he needed the patience of a saint to beat world number 13 Sinner, a talented 20-year-old with a huge future in the game.Unfortunately for Sinner, 35-year-old Djokovic proved yet again he is not ready to be consigned to the past.There was no hint of the drama to come when Djokovic raced into a 3-0 lead but a sloppy service game with two double faults, including one on break point, allowed Sinner – who had never won a tour level match on grass before these Championships – to draw level.The Serbian saved a break point in his next service game but Sinner struck again for 6-5 with a whipped forehand winner and served out the opening set.Worse was to come for Djokovic, as when Sinner successfully challenged a ball on the baseline the six-time champion was suddenly a set and two breaks down.As Djokovic floated a backhand long, Sinner took the second set having won 100% of the points behind his fizzing first serve.But just as he looked on the cusp of unravelling, the top seed rediscovered his range and hit back in the third.Late in the fourth Sinner took a nasty