World number seven Juan Martin del Potro heads the Tokyo field in the absence of Wimbledon champion Andy Murray, who pulled out to undergo minor back surgery.
The giant Argentine, US Open winner in 2009, faces Cyprus's Marcos Baghdatis on Tuesday in his opening match at the venue for the 2020 Olympic tennis competition.
Tsonga and Monfils live close to each other in Switzerland and often socialise together, but the French connection counted for little once they stepped on court as Tsonga -- frequently likened to Muhammad Ali because of his looks and physique -- overpowered his compatriot with a brutal display of hitting.
The world number eight, still feeling his way back from a knee injury, bulldozed his way to a 3-0 lead before taking the first set with a vicious serve which was too hot for Monfils to handle.
Monfils, brightening up centre court with his brilliant orange attire, began badly off-key but made more of a fist of the second set, forcing a tiebreak.
However, Tsonga, the 2009 Japan Open champion, simply had too much firepower and a fourth win in five meetings was never in serious doubt.
Having frittered away his first three match points in the breaker, Tsonga clinched it 10-8 with a classic knockout blow, a huge forehand into the corner setting up a simple put-away volley after one hour, 31 minutes.
Tsonga retired from the second round at Wimbledon and only returned at Metz a week ago, reaching the final. After wrapping up victory over Monfils, Tsonga immediately grasped his left knee and winced.
In other matches, sixth-seeded Spaniard Nicolas Almagro beat qualifier Benjamin Becker 7-6, 7-6 while Ukrainian Alexandr Dolgopolov completed a bad day for Germany with a 6-3, 6-1 thrashing of Daniel Brands.
Local hero Kei Nishikori begins the defence of his Tokyo title against Austria's Jurgen Melzer on Tuesday. (AFP)
