LONDON: Juan Martin del Potro admitted he almost quit Wimbledon after suffering a horror fall in the first game of his quarter-final against David Ferrer on Wednesday.       Del Potro, the Argentine eighth seed, was in tears after his already-injured left knee bent awkwardly in the fifth point of the match on Centre Court.       The 24-year-old originally injured the knee during his third round win over Grega Zemlja and the pain was so excruciating when he fell against Ferrer that he was on the verge of pulling out.       "I think I was close to retiring," Del Potro said. "But to be honest, I didn't want to retire in my first Wimbledon quarter-final. And that's the reason for continuing to play.       "The doctors gave me good anti-inflammatories. I broke his serve early and that gave me confidence to take advantage in the beginning of the match."       Despite taking painkillers, Del Potro was clearly struggling to move at times, but he gritted his teeth through two hours and 16 minutes of gruelling action to reach his first Wimbledon semi-final as the former US Open champion defeated Spanish fourth seed Ferrer 6-2, 6-4, 7-6 (7/5).       "I was worried because the doctor said they can't do any more with my knee," Del Potro said.       "I had the tape, a very tight tape, and that helped me to move a little bit, but nothing more.       "It was really painful in the beginning of the match and many things came to my mind, many bad things.       "But I tried to be positive and played unbelievable tennis. I was lucky because I did everything good."       Del Potro's reward for his heroic efforts is a daunting clash against world number one Novak Djokovic on Friday for a place in Sunday's final.       It will be del Potro's first Grand Slam semi-final appearance since his breakthrough triumph at the US Open four years ago.       Del Potro is just the second Argentine man to feature in the Wimbledon semi-finals, following in the footsteps of David Nalbandian in 2002.       Del Potro took the bronze medal at last year's London Olympics, played at the All England Club, after beating Djokovic in the third place match.       But the Argentine knows he will be a heavy underdog against a relentless competitor like Djokovic unless he can make a quick recovery from the injury.       "I will need to be 110 percent against him. He's the number one. He's a former champion. It's going to be a more difficult match for me," Del Potro said.       "I will see tomorrow how I am. I think the bad pain is coming now after the shower, after the massage.       "But the doctors are taking care of me. I'm safe. I think I will feel the same as after my third round match. I need to be strong to manage the pains."       Del Potro added: "I need to practise tomorrow for sure, but I'm positive and I'm not thinking too much in my knee."       "I'm not going to put my body at risk. The doctors tell me with this tape and taking some antiinflammatories you can play. If they say something different, I will think about it.       "But I have experience about injuries. I have my knee problem, but always the opponent can have different injuries.       "If I'm okay, if I do everything good to be ready for my next match.       "I will be excited to play against him. I remember the match during the Olympic last year on the same surface."