"Couldn't have picked a more beautiful & perfect song to start the year with," Michele, 26, Tweeted Monday.
She also wore a special necklace, a simple silver medallion on a chain. The ornament reads "Cory."
Filming on the new season of the FOX show was postponed until this week, and while the first two episodes of season five will be Beatles-themed, the third episode will be a tribute to Monteith.
The delay also gave the cast and crew time to deal with the loss of Monteith, who died in a Vancouver hotel room from a toxic combination of heroin and alcohol.
Because the soundtrack for every episode is recorded in advance, it's believed that Michele might have been prepping songs on Monday for this special episode.
While it will be revealed what Michele sang when the show airs Oct. 10, executive music producer Adam Anders did Tweet to her on Monday, "you are the strongest person I know. That was truly amazing. Love you xoxo #leastrong."
Alex Anders, Glee's vocal producer and recording engineer, also Tweeted Michele a message, saying, "You were absolutely magnificent today. I am in awe of your strength and your sweet spirit. Love you. #leastrong."
Proceeds of music sales from that episode will go toward a fund in Monteith's name.
Fans of Monteith may make donations in his name to three charities that were especially important to him: Project Limelight Society, which exposes youth living in Vancouver's Downtown Eastside to the arts; Virgin Unite, founded to support entrepreneurial efforts to better the world; and