Ant, 41, took some time off a year ago, including missing the latest I'm A Celebrity series, after he was convicted of drink-driving and went to rehab to face his demons.
MAY 28: Dec hosts Britain's Got Talent live shows by himself.
McPartlin took a break from presenting duties following a drink-driving arrest.
GMB co-host Ben Shephard admitted Ant is "looking well", and her fellow presenter Susanna Reid joked he will still "clean up" at next week's National Television Awards, where Ant and Dec Donnelly are aiming to triumph in the Best Presenter category for an 18th consecutive year.
"Ant welcome back. We missed you; it wasn't the same".
Ant McPartlin broke down in tears when he appeared onstage for the first time after a year away from the spotlight.
After he missed I'm A Celebrity. Oh God, I'm really emotional now.
"With the greatest respect to the boys", he continued, "I think Holly has had such a great year, and Graham".
Nishikori back from brink to overcome Karlovic in thriller
But she made amends in the next game and saw off two break points from Zidansek before bringing up two match points on serve. I was down 7-6 in a tiebreaker, but I returned well and I focused well over the last couple of points.
Donnelly later fronted I'm A Celebrity...
REUNITED: Ant and Dec were happier than ever to be reunited (Pic: ITV) BACK TO WORK: The producers weren't sure if Ant would be returning (Pic: ITV) "Shall we move on before I cry?"
Simon Cowell had the idea for "Britain's Got Talent" while watching a rival music show. I would be happy but not as happy if that makes sense.
Ant and Dec have posted a picture of themselves on their first day working together for nearly a year.
He added: "So on behalf of all of us, welcome back, we're family".
Following McPartlin's drink-driving charge, he was banned from driving for 20 months and fined £86,000.
Ant, who divorced wife Lisa last year, previously struggled with a two-year addiction to super-strength painkillers following a knee operation.
Britain's Got Talent 2019 will start on TV later this year.