Goffin faces Diego Schwartzman in Saturday's semi-finals after the Argentine beat Steve Johnson 6-0 7-5 to reach his second ATP World Tour semi.
Despite suffering a scare in the second set when Mannarino - fresh from his semifinal win over top seed Marin Cilic - found himself a break up, the No. 4 seed recovered to claim both the set and his fourth career title.
Gasquet has not troubled the business end of the Grand Slam tournaments this year, losing in the first round of Wimbledon and the US Open and getting to the last 32 in his home tournament at Roland Garros as well as the Australian Open.
Goffin was made to work hard against Gasquet, who led 3-2 when rain stopped play and, with the players remaining on court, the Ariake Colosseum roof began to close.
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Goffin, who lost in last year's final, has been on a winning run in Asia. "I needed one or two games just to get used to the conditions, then I started to feel the ball really well, feel my rhythm, become more aggressive game after game".
This was the 9th win in a row for the Belgian and with these 500 points he moves to 7th place in the ATP Race with 2740 points, looking good to book the place in London for the World Tour Finals. With Novak Djokovic and Stan Wawrinka out for the season, 10th place will be enough to secure a place at the season-ending tournament. "To play like this is a very big boost to my confidence".
In a Franco-Belgian clash that served as a foretaste of next month's Davis Cup final, Goffin found himself 5-3 down and facing three set points against the former world number seven before coming through 7-5, 6-2.
Canadian Milos Raonic's injury woes continued on Thursday as he was forced to retire from his second-round match against local favourite Yuichi Sugita at the Japan Open.