Andy Murray says he feared injury would bring a premature end to his French Open campaign, as he bids to beat David Ferrer on clay for the first time.After overcoming a back injury to ease past Richard Gasquet, Murray heads into Wednesday's quarter-final 5-4 up in past matches with the Spaniard - but has lost their three meetings on clay."A few days ago I thought I was done so to get this far is great," Murray said."I've always found it tough against him on clay but I feel I'm playing well."Having been troubled by a back spasm during his win over Jarkko Nieminen in round two, the 25-year-old's injury worries appeared to have resurfaced when he was left clutching his hip as Gasquet easily won the first set on Monday, only for the Scot to rally and clinch a 1-6 6-4 6-1 6-2 triumph.Although history is not in Murray's favour against Ferrer, the pair have not played each other on clay for two years and, since that meeting in Madrid, the world number four has won four of their five clashes on hard courts."Every match is different," said Murray. "A lot of the clay courts play differently - quicker, slower. Conditions change things. We'll just see whether having lost to him a few times on the clay before is a factor or not."Six-time French Open winner Rafael Nadal, who faces Nicolas Almagro, is likely to await the winner in the last four, but Murray, the only non-Spaniard left in the bottom half of the draw, insists talk of a semi-final rematch against the reigning champion is premature.He said: "I think he (Ferrer) is one of the toughest guys to play on any surface. He's number six in the world, and he's been there for a long time now. He's had a good clay-court season so far."We know each other's games very well, and he's one of the best players in the world on any surface, so it's going to be a tough match."Ferrer, seeded sixth, is yet to drop a set in Paris this year, however he expects a stern test against a player he believes could have topped the world rankings."I think he (Murray) could have been number one because he analyses matches really well. He reads the opponent's game very well."He's a player whose first serve is outstanding. He's very gifted and talented. He moves well to the net, his volleys are really excellent. He's a very tough player to play against."