Writing "F" for "Federer wins the set" and "R" for "Roddick wins the set" we can have

"RRR" (Roddick wins in three straight sets)

"FFF" (Federer wins in three straight sets)

"RFFF"

"FRFF"

"FFRF"

(But NOT "FFFR" since that would end at "FFF".

"FRRR"

"RFRR"

"RRFR"

"RRFFF" etc (there are 5!/(2!3!)= 10 ways to order 2 Rs and 3 Fs but we do NOT include FFFRR or FFRFR, etc. There are 4!/3!= 4 ways to get 3Fs in the first four sets so we subtract 10- 4= 6.)