Hollywood heavyweights MGM and Sylvester Stallone are hoping the sixth round of the Rocky saga delivers a box-office knockout.

MGM said yesterday it has signed Stallone to write a Rocky VI screenplay with an update to the story of the scrappy Philadelphia palooka who becomes the champion of the world.

So far, details were sketchy because Stallone -- who wrote and directed 1976's original Rocky -- has just been hired.

But the intention is to avoid a big-budget sequel and return to the gritty feel of the original.

Early plans for Rocky VI has the 56-year-old Stallone portraying Rocky Balboa once more. This time, Rocky is running a youth center when he's lured out of retirement for one last fight.

The first Rocky was a labor of love for Stallone. Made for a little more than $1 million, Rocky raked in an astounding $220 million in worldwide ticket sales. It earned critical acclaim and won the Oscar for Best Picture.

The sequels all grossed over $100 million in ticket sales -- except for the final installment, Rocky V in 1990, which took a dive with a pathetic $40 million gross and a flurry of critical jabs from disgusted critics.

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