A back complaint has forced Brisbane International feel-good story Reilly Opelka to retire in the first set of the final, handing Czech Jiri Lehecka the title.

Opelka missed nearly two years on tour after wrist and hip injuries but, after returning last July, was on the cusp of a brilliant comeback title that included a defeat of Novak Djokovic on the way to the decider.

A lingering back injury got the better of the American giant on Sunday though, Opelka attempting to play through the pain before retiring down 4-1 in the first set.

"Thank you and sorry at the same time," Opelka said.

"It's been a great week for me ... got a lot of great matches in here.

"The tournament doctors and physician were unbelievable. They threw the kitchen sink at it (his back) all day but it wasn't right to compete."

Lehecka, who beat third seed Holger Rune in the first round, progressed to the final after defending champion Grigor Dimitrov retired from their semi-final on Saturday.

"I know how you feel Reilly. It's been an honour to see you back on tour," Lehecka, who won the Adelaide International last year, said.

"It was a great week here for me in Brisbane. I just love to play here in Australia."