Olympic champion Alexander Zverev continued his tune-up for the Paris Games by moving into the semi-finals of the ATP clay-court tournament in his home city of Hamburg. The 27-year-old beat China’s Zhang Zhizhen 6-4, 6-3 on Friday as he seeks to defend his Hamburg crown and meets Pedro Martinez in Saturday’s last four after the Spaniard battled past Argentina’s Francesco Cerundolo 1-6, 6-1, 6-4. Zverev, the world number four, had looked doubtful for Hamburg and the Paris Olympics (July 26-August 11) with a knee injury suffered at Wimbledon. But he was given the all clear and is making of most …