Nadal will play in the second match of the day scheduled for Center Court as he faces Philipp Kohlschreiber in their opening round match. Federer is scheduled to play Nikolay Davydenko following Nadal's match, an interesting first-round matchup as Federer and Davydenko actually met in the championship final last season.
The entire Order of Play for Center Court at ATP Doha is listed below for January 3rd:
CENTER COURT start 3:30 p.m.
M. Jaziri vs  Jo-Wilfried Tsonga
 Rafael Nadal vs Philipp Kohlschreiber
Nikolay Davydenko vs  Roger Federer
 Gael Monfils vs Rui Machado
Two key players in action on Tuesday who are not on Center Court include Ivo Karlovic and Mikhail Youzhny. They will be playing their first-round matches on Court No. 1.
Federer is considered the betting favorite to win in Doha while Nadal is considered the second favorite (source: bet365). Both players will be looking to gather some momentum following disappointing recent efforts in an exhibition event in Abu Dhabi.
With the 2012 Australian Open just a couple of weeks away both players will need to hit their best tennis in order to challenge for the first Grand Slam event of the calendar year.