The singer, 20, rejoined his bandmates during their appearance at Capital FM's Summertime Ball to a crowd of 80,000.
Sykes apparently suffered from a haemorrhaging vocal chord, a condition Adele also suffered from in 2011, which meant he had to undergo emergency surgery in Los Angeles in April.
The singer quipped that he'd miss watching daytime TV and playing computer games during an interview after his comeback performance.
"I'm going to miss my days of Jeremy Kyle, This Morning and the PlayStation, but it's great to be back," he said.
He added: "Obviously I've spent time getting back to where I was before. But yes, I should probably work hard and get up to date."
Max George also said that it felt "pretty awesome" to have his bandmate back in the fold.
"We've missed him. We've been slogging it out for over two months now as a four.
"We have a lot less to do now he's back because he's quite heavy with the vocals which is cool for the rest of us."
Robbie Williams headlined the one-day event yesterday, which also included performances from Psy, Taylor Swift, Will.I.Am, Jessie J, The Saturdays, Olly Murs and Labrinth.