To solve this question, the user needs to know a little bit about the history of Roger Federer in tennis.

Roger Federer is a Swiss tennis player who has won 20 Grand Slam singles titles, the most in history for a male player. He won his first Grand Slam title at Wimbledon in 2003.

Now, let's go through each option and explain why it is right or wrong:

A. Andy Roddick - This option is incorrect. Roger Federer defeated Andy Roddick in the final of Wimbledon in 2004 and 2005.

B. Lleyton Hewitt - This option is incorrect. Roger Federer defeated Lleyton Hewitt in the quarterfinals of Wimbledon in 2004 and in the final of the US Open in 2004.

C. Mark Philippoussis - This option is correct. Roger Federer defeated Mark Philippoussis in the final of Wimbledon in 2003 to win his first Grand Slam title.

D. Andre Agassi - This option is incorrect. Roger Federer defeated Andre Agassi in the quarterfinals of the US Open in 2004.

Therefore, the answer is: C. Mark Philippoussis