Here's the full line-up for Group A and Group B for singles and doubles. Personally I think both groups in the singles round robin are equally tough.
Let's see who will FTW!
Singles round robin groups for the ATP World tour finals:
Juan Martin del Potro
Doubles round robin groups for the ATP World Tour finals:
Daniel Nestor/Nenad Zimonjic
Mahesh Bhupathi/Mark Knowles
Frantisek Cermak/Michal Mertinak
Mariusz Fyrstenberg/Marcin Matkowski
Bob Bryan/Mike Bryan
Lukas Dlouhy/Leander Paes
Lukasz Kubot/Oliver Marach
Andy Ram/Max Mirnyi
(Images via Yahoo! Sports)