Wawrinka took out Jeremy Chardy 6-4, 7-6 (7-2), winning three-quarters of the points on serve. He was especially effective in the first set, dropping just six points when serving.
Each player had a second-set break but Wawrinka, the only seed still in the tournament, came out ahead in the tiebreaker and improved to 4-0 lifetime versus Chardy.
He'll go against Guillermo Garcia-Lopez in the first of Friday's semifinals. Garcia-Lopez defeated qualifier Jay Hernych 6-4, 6-1 behind five breaks.
Wawrinka had a 6-1, 6-2 win over Garcia-Lopez this year that gave him a 3-2 advantage in career meetings.
Mahut took a tight three-set match from Malisse in 2006, in their only previous meeting.