By Frank Pingue

(Reuters) - NBA teams will open a new season this week with optimism abound but it may be false hope for all but the Cleveland Cavaliers and Golden State Warriors who look headed for a third consecutive title clash.

LeBron James, who four months ago led the Cavaliers on an improbable comeback over the Warriors in the NBA Finals, will once again power a Cleveland team that are the class of the Eastern Conference by a considerable margin.

Repeating as champions is one of the most difficult things to do in sport and while Cleveland, who open the 2016-17 season on Tuesday, will face some spirited competition, they likely won't truly be tested until the latter stages of the postseason.

The biggest perceived threat to the Cavaliers' reign in the East could be the same Toronto Raptors team they needed six games to eliminate in last season's conference final.

The Warriors, who were already flush with talent when they posted a record-setting 73-9 mark last season, have since added four-time scoring champion Kevin Durant to form what some consider the most lethal roster ever assembled.

But Golden State will no doubt have a target on their backs each night and perhaps the only thing that could get in their way of representing the Western Conference in the Finals will be the overwhelming expectations.

Golden State did well to weaken one of their challengers by signing free agent Durant, who led the Oklahoma City Thunder to a 3-1 series lead over the Warriors in last season's conference final before an epic collapse.

But Golden State will by no means have an uncontested road to the Finals and many feel the San Antonio Spurs have the best shot at beating the Warriors should they clash in the postseason.

With Tim Duncan having announced his retirement, the Spurs are embarking on a season without the long-time face of their franchise for the first time since 1996.

The Spurs replaced the five-time NBA champion with Spanish power forward Pau Gasol, who along with Kawhi Leonard, LaMarcus Aldridge and evergreen veterans Tony Parker and Manu Ginobili should make sure San Antonio remain a threat in the West.

While last season showed that nothing is certain in sports, the Warriors and Cavaliers are clearly in a league of their own and ideally positioned to deliver a mouthwatering battle for NBA supremacy in eight months.

(Reporting by Frank Pingue in Toronto; Editing by Mark Lamport-Stokes)