The two players have met three times, all in matches that took place in 2012. Their head to head encounters were all competitive however Federer took all three away, one on grass, one on blue clay and one on the hard court surface. Raonic and Federer have not yet met in a best of five-set match but if anything, the change of format should favor the World No. 2.
Federer is clearly the better conditioned athlete and that factor often makes a decisive difference in long matches. You have to think that if Raonic were to challenge Federer for a spot in the quarters, it would take several hours to push the Swiss Maestro to the brink. You also have to think that any match that takes more than 3.5 hours or more should sharply favor Federer due to his superior conditioning.
Raonic is not likely to be intimidated in the fourth round, partly due to the fact that he has natural confidence and partly due to the fact that he's played Federer tough in Masters Series events. Their last two matches both went to deciding set tiebreakers and the Canadian enters their Melbourne Park contest knowing that it was mere points that prevented him from beating Fed in the past.
However Raonic has never won a fourth round match in a major before and Federer has survived the fourth round in the each of the last 34 Grand Slams. As the 13th seed in the event, there's really not much pressure on Raonic to prove anything on Monday with his best hopes for victory resting on brilliant serving and victories in tiebreakers.
Given Federer's own service skills and given his proven prowess in tiebreakers it's hard to envision an upset of the World No. 2 coming up in the fourth round.