I thought it was a stranng choice too, but in the newspaper I read Martens chose Christophe because he has a game that can surprise the other player while Olivier has a classic game. (in't Nederlands: Christophe kan het spel van zijn tegenstander echt ontwrichten en ze uit hun evenwicht brengen, terwijl Olivier een veel klassieker spel heeft volgens hem). Also because Christophe has had some good results against top-Spaniards: his head to head against Costa is 1-0, against Corretja 2-0 and 0-0 against Ferrero and Moya!
(so he hadn't lost against any of the Spaniards in the team until now)
I hope they can win their doubles match tomorrow and then surprise everyone by winning both Sunday singles matches too!!!