Grenier auditioned for series one and two of Tough Enough, the reality television program produced by World Wrestling Entertainment in 2001 and 2002, but was unable to attend the selection process due to visa problems.
During this period, Grenier made two appearances for WWE, the first at No Way Out in 2003 as a referee during the match between The Rock and Hulk Hogan in which he helped The Rock to win that match; the other at WrestleMania XIX during the match between Hogan and Vince McMahon.
On April 28, Grenier returned to WWE, debuting on the Raw brand alongside fellow French-speaking Canadian René Duprée. The duo were collectively known as La Résistance, and were billed as Frenchmen who were highly critical of American efforts in the War on Terror. Grenier and Duprée made their first appearance on the April 28 episode of Raw as La Résistance by attacking Scott Steiner who had recently made remarks comparing France to hell, offending Grenier and Duprée. La Résistance went on to feud with Scott Steiner and Test (who was being forced to be Steiner's tag team partner by Stacy Keibler) until the Judgment Day pay-per-view where La Résistance defeated Steiner and Test.
Grenier and Duprée went on to win the World Tag Team Championship from Kane and Rob Van Dam at the 2003 Bad Blood event. After that the decision was made to add Rob Conway as a third member who began by posing as an American serviceman being abused by Grenier and Duprée until the Dudley Boyz came out to save him. Once the Dudley Boyz had their backs turned to Conway, he attacked them with the American flag they had given him before tearing it off the pole and laying it on top of them. The trio began carrying the French flag to ringside and singing the French national anthem before their matches. During their time together the trio feuded with several tag teams; including the Dudley Boyz, The Hurricane and Rosey, and Garrison Cade and Mark Jindrak.
They held the titles until Unforgiven 2003 when they dropped them to The Dudley Boyz in a three-on-two handicap tables match. During the match suffered neck and back injuries, and was diagnosed with a broken neck a month later. The trio's eventual reunion was on March 15, 2004 when Grenier. This reunion only lasted a week, as Dupree was drafted to SmackDown! on March 22, 2004 during the Draft Lottery.
Without Dupree, Grenier and Conway continued to team together as La Résistance on Raw. With American-Franco hostility subsiding, WWE ceased billing the team as being from France and instead promoted them as being haughty French Canadians who carried the flag of Quebec to the ring, with Grenier often singing the Canadian national anthem.
The team held the World Tag Team Championship three times. Their first title reign began when they defeated Chris Benoit and Edge on the May 31, 2004 episode of Raw in Grenier's home town of Montreal. They dropped the championship to them at Taboo Tuesday when Benoit was able to defeat both Grenier and Conway, despite Edge abandoning Benoit during the match. La Résistance re-captured the championship on the November 1 episode of Raw when Edge walked out on Benoit again. Two weeks later, however, in a three-way tag team elimination match, William Regal and Eugene were the last men standing, beating both La Résistance and the team of Tajiri and Rhyno. Their final Tag Team Championship win occurred at a Raw house show in Winnipeg, Manitoba, Canada on January 16, 2005. La Résistance defeated William Regal and Jonathan Coachman (who substituted for Eugene due to suffering an injury). La Résistance dropped the championship a few weeks later on the February 7, 2005 episode of Raw in Tokyo, Japan to William Regal and his new tag team partner Tajiri.
La Résistance repeatedly attempted to regain the title, mainly on Sunday Night Heat, but fell short during each one. La Résistance were given another chance at Regal and Tajiri during a Tag Team Turmoil match involving five teams during Backlash. In the match, La Résistance managed to eliminate the current champions but were defeated by the last remaining team, Hurricane and Rosey.
The team split in mid-May 2005 after repeatedly quarreling over which of them was the superior wrestler. Grenier and Conway went into singles competition with announcers claiming that the members of La Résistance were trying to "one up" each other in singles matches without the other man in their corner. This saw Conway defeat a jobber and Val Venis, while Grenier was defeated by both Val Venis and Chris Jericho before both men competed in a triple threat match (with Intercontinental Champion Shelton Benjamin for the title), Benjamin won the match after the La Résistance members began to argue and fight with each other. The altercation between the two led to a main event match for the June 12, 2005 edition of Sunday Night Heat. This match saw Conway portray the heel, and defeat Grenier.
Grenier was a last minute trade in the 2005 WWE Draft, forcing him to leave Raw and go to SmackDown ending his already shakey tag team partnership with Conway. After being traded he was not seen on TV for a few months, eventually re-debuting on SmackDown on September 1, 2005. He was repackaged with a gimmick of a narcissistic male supermodel known simply as "Sylvain"", though his name was later tweaked to "Sylvan". His new gimmick saw him speak with a light French accent, wear sunglasses and use techno music for his entrance.
Sylvan soon began a brief feud with Hardcore Holly, which saw Sylvan determined to protect his "perfect face" at any cost. Sylvan gained a victory over Holly on the October 7 episode of SmackDown with help from Mr. Kennedy. At No Mercy 2005 after Kennedy defeated Holly, Sylvan came out and continued his feud with him by attacking the injured Holly, which led to Sylvan losing a singles match on an episode of Velocity and a Hardcore Street Fight the following week on SmackDown, ending an almost two-month feud. Sylvan entered the 2006 Royal Rumble, but was quickly eliminated by Bobby Lashley. After this, Sylvan slowly reverted to using anti-American tactics while still retaining his model gimmick. However, this change was short-lived and he was soon taken off of television, where he began wrestling in dark matches before SmackDown! tapings.
On the June 16, 2006 episode of SmackDown vignettes designed to look like vacation promotion videos for Québec and starring Sylvan ("the Ambassador at Large for the Beautiful Province of Québec", or "the Ambassador of Québec" for short) began airing. On July 7 he appeared in-ring to cut a promo attempting to "make peace" with the US fans by welcoming them to visit his home province. The fans responded with a chorus of boos and a loud "USA" chant. In the coming weeks he continued to appear backstage, telling everyone within earshot how much better Québec was than where ever they were from. He made his ring return with this gimmick on the July 28 SmackDown with a win over Tatanka. Sylvan would go on to job against the likes of Tatanka, Vito, and Batista under this gimmick. Sylvan's losing streak ended on the first CW Network episode of SmackDown on September 22, 2006, when he defeated Tatanka again by using the ring ropes for illegal leverage. The following week on SmackDown, Sylvan defeated Jimmy Wang Yang, who had insulted Canada the week before. The next week, Sylvan cheated again to take a lead in the feud. The following week, Sylvan finally lost to Yang in a six-man tag team match. After disappearing for a couple of weeks, Sylvan finally returned to cost Yang a match versus Gregory Helms. After that match, Matt Hardy came out to help Yang ward off Helms and Sylvan. The following week, Hardy and Yang defeated Helms and Sylvan, ending Sylvan's feud with Yang.
On the December 22 episode of SmackDown!, he disguised himself as Santa Claus and attacked World Heavyweight Champion Batista with a steel pipe. This resulted in a match with Batista that Sylvan quickly lost and would end the "Ambassador of Québec" gimmick as Sylvan disappeared from television after this.
On the February 20, 2007 episode of ECW, Grenier returned and reformed the original La Résistance with René Duprée, defeating a pair of local wrestlers. Due to Duprée getting suspended at the start of March, however, and being sent to rehab after violating the Health and Wellness policy, the team was quietly disbanded. At the August 7, 2007 taping of SmackDown!, Sylvan made his televised return under his old model gimmick in a match against Kane, which he lost. This would turn out to be Grenier's final match for WWE, as he was released from his contract six days later.