 During the summer , he experimented with various different racquets and played the German Open with a <unk> @-@ out 98 @-@ inch Wilson racquet , instead of his regular Pro Staff 6 @.@ 1 90 <unk> racquet with the smaller 90 @-@ inch hitting area . He returned to his regular racquet for the second half of the season . After Wimbledon , Federer continued to be upset early in tournaments because of a serious back injury through October , when he announced that he was parting ways with Paul <unk> , his coach for the last three years . Federer made the final in Basel , succumbing to Juan Martín del Potro . 
