The 2009 MotoGP World Championship suddenly became a two-rider fight after Ducati Marlboro's Casey Stoner was forced to withdraw from the next three races with continuing medical problems.In a team press release, Stoner said, "After five extremely difficult races due to my health, I returned to Australia to visit the sports doctors who have looked after me for many years. We have taken the difficult decision not to contest the next three rounds of the championship, to allow my body time to recover from the recent stress."Stoner first revealed the fatiguing illness following his third place finish in the Catalunya Grand Prix in late June. The condition didn't approve at the following Dutch TT in Assen, nor at the Red Bull U.S. Grand Prix. After Laguna Seca, Stoner was extensively tested by Dr. Arthur Ting, who diagnosed the Australian with slight gastritis and mild anemia. But when the series resumed two weeks later in Germany, Stoner continued to suffer. And it didn't improve in the British GP, the last race prior to the three week summer break.After Donington, Stoner flew home to Australia for further tests with his personal physician. It was after that visit that it was decided that he sit out the races in the Czech Republic, Indianapolis, and Misano, before returning for the Grand Prix of Portugal on the first weekend in October. Stoner will be replaced in Brno by Pramac Racing's Mika Kallio, who will team with first year Ducati rider Nicky Hayden."The doctors believe that during the Barcelona race I was suffering from a virus, and, that I subsequently pushed my body too hard, leading to problems that have caused my fatigue since then," Stoner said. "The doctors are continuing with many tests to try to understand these problems and make sure it does not happen again. I have spoken with Ducati and thank them for their understanding at this time. I feel very sorry for the factory, my team, my sponsors and the fans and I am also disappointed because the bike in the last races has been very competitive. I will be doing everything possible to come back at full strength for Portugal."Livio Suppo, the MotoGP project director said that it was "obviously difficult for us to contemplate the next three races without Casey, but it's clear that the most important thing at this time is his health. Since his debut with us in Qatar in 2007, Casey has won 18 times, as well as taking countless podium finishes, becoming one of the top riders in the World Championship in the process. We have already overcome tough times together, like when he had to undergo surgery on his scaphoid towards the end of last season. We are united whether in times of success or in times of difficulty."Suppo voiced confidence that the condition would be resolved for the end of the season, which would permit Stoner to prepare for the 2010 season. Meanwhile, the young Finn Kallio will step up to the factory team. Kallio began his rookie MotoGP campaign with promising eighth place finishes in the first two races, in Qatar and Japan. But he hasn't been able to match those finishes. In the following eight races he's had four non-points paying finishes, with a best result of ninth in Catalunya, and sits 15th in the World Championship.For Brno, Kallio's seat at Pramac will be taken by Ducati Xerox World Superbike campaigner Michel Fabrizio, who went on summer break following the WSB race in Brno. No replacement was named for the Red Bull Indianapolis Grand Prix, which is an open weekend for World Superbike, nor Misano, which conflicts with the Nurburgring WSB race. Kallio's Pramac Racing teammate Niccolo Canepa has MotoGP experience on the Indy track, having taken part in last year's test prior to the inaugural Indy GP."When I took the call I could hardly believe it, but then I talked to my manager and I realized it was true," Kallio said. "Obviously, I'm not expecting to suddenly be a second quicker, because our bikes are very similar to the official machines, but I hope to be able to pick up some good results for the team."Nicky Hayden suddenly finds himself leading the team and without Stoner, who was only 12 points out of second place in the World Championship. This will be Hayden's first race in Brno since 2007. The Kentuckian was forced to sit out last year's Czech GP after injuring his foot in the X-Games Supermoto race."I can't wait to get back on track and score some results which will make my team happy," Hayden said. "I hope Casey can get better over the next few weeks, because we all know how quick he is when he is fit. In the meantime, I need to try to make progress with the team and try to make sure that they don't miss him too much."The MotoGP title fight now centers on Fiat Yamaha teammates Valentino Rossi and Jorge Lorenzo. The Italian leads the Majorcan 187 to 162 after 10 of 18 races. Stoner is third at 150, with Dani Pedrosa's (Repsol Honda) 72 point deficit to Rossi too much to make up in the final seven races.