Lewis Hamilton revealed he had his reflexes to thank for avoiding a massive high-speed accident during Friday practice for this weekend's Belgian Grand Prix. The 26-year-old McLaren driver said that he managed to correct the movement of his car after it hit a 'huge river' of water running across the spectacular, but dangerous, Spa-Francorchamps circuit. "I must have been doing 150 mph (240kph) and I had a huge 'tank-slapper," Hamilton said. "There was a huge river right the way across the track and I am lucky I was awake to correct it or it would have been all four wheels, the front wing and the rear wing. (broken off the car). I'm so glad I was on form." The near-miss happened on Friday morning during the opening free practice session at the Blanchimont corner. The Briton, world champion in 2008, said he was wary about forecasting what may lie ahead on Saturday and Sunday as more heavy rain is forecast for the Belgian Ardennes. He added that it would be very difficult to choose the right levels of down-force and the gearing levels needed to seize victory. "Even before the weekend we knew it was going to be difficult because the Drag Reduction System (DRS) makes such a difference," he said. "You get past sixth gear and into seventh and in some cases, when DRS isn't engaged, you don't then get out of sixth. "Also, with the fuel loads much heavier for the race, then it's going to make a massive difference. Setting up the car for qualifying is one thing, but making sure you don't lose too much time in race conditions without DRS is a big thing." Hamilton was fifth-quickest in the morning session before improving to fourth behind his McLaren team-mate and fellow-Englishman Jenson Button, the 2009 world champion, during the afternoon. Hamilton had arrived in Belgium trying to shake off a slight illness following a summer holiday in the United States with his fiancee, the singer Nicole Scherzinger. "I was feeling refreshed, but I caught a bug when I got back," he said. "So, it wasn't the greatest, I'm just recovering from it."