For Parisians, Boulogne is the other end of the earth. OK, so it's at the end of the line but fear not, there are two options for arriving:
Line 9: Mairie de Montreuil - Pont de Sèvres (stations: Michel-Ange Auteuil, Michel-Ange Molitor).
Line 10: Gare d'Austerlitz - Boulogne (stations: Porte d'Auteuil or Boulogne Jean-Jaurès).
Trains runs every 5 minutes on average and be smart either buying a day pass or a return ticket in the morning.
When you the stadium things will reach peak pushy-ness, so it's best to walk farther to Michel-Ange Molitor station (on both lines 9 and 10) to avoid the queues that often build up at other stations.
For full information on getting to Roland Garros, head to the site.