Sellas Tetteh has warned his players to keep it clean in their quarterfinal meeting against South Korea at the Under-20 World Cup after the Black Satellites picked up two red cards in their last two games.Tetteh says he is worried about the red cards and the quality of refereeing in those two decisions but says it will be important for the players not give referees any cause for bad calls. Rabiu Mohammed was sent off for Ghana against Uruguay in the final group game before Opoku Agyemang was given his marching off orders in extra-time of the second round meeting against South Africa. On both occasions, replays showed the referees had got it completely wrong but Tetteh says there is no point complaining about the referees. "The red cards have worried me largely because on both occasions the players didn't deserve it," Tetteh said. "But the referees are human and make mistakes and what I can do from my end is to ensure my players don't give them any reason to." Tetteh is looking to take Ghana into the semifinals for a fourth time in five appearances and reckons the Koreans will be more than a match for them. "They are fast, swift and with great ball control. We will need to be at our best against them but we have showed we can face any team. We have showed at our best we are not only capable of winning but playing beautiful football. We are not favourites yet even with Spain and Egypt out. At best, we are still best outsiders."Source: Kickoff.com