By Ian Ransom

MELBOURNE (Reuters) - The Richmond Tigers start their bid for a fourth Australian Football League championship in five years on Thursday as fans welcome the indigenous sport back to its Melbourne heartland after a pandemic-blighted 2020 season.

Authorities approved a half-capacity crowd of 50,000 at the Melbourne Cricket Ground for Richmond's clash with the Carlton Blues, the traditional season-opener of the Australian Rules top flight.

The match marks the return of 'footy' to AFL-mad Melbourne, which last saw a game in July when Richmond defeated the Demons in the fifth round of the 2020 season, a game played in front of empty terraces at the MCG due to social distancing restrictions.

A second COVID-19 wave in Victoria chased the 18-team competition out of the southern state in July and forced Melbourne's nine teams and the Geelong Cats to play out the rest of the season in interstate hubs.

Despite the disruptions, Melbourne-based Richmond ended up meeting the Cats in the "Grand Final" championship decider in Brisbane in October, with the Tigers defending their title in a 31-point win.

Richmond will bid to become the first team to claim a hat-trick of AFL premierships since cross-town rivals Hawthorn Hawks completed the 2013-15 treble.

A fourth flag in five years would also make Richmond the most dominant side since the competition expanded out of Victoria and went national in the mid-1980s.

As for all professional sports leagues, the AFL's finances were battered by COVID-19 and the league will hope crowds can help their balance sheet recover.

Australia has virtually eliminated the virus but authorities have been quick to launch snap lockdowns when cases of community transmission surface.

NEW RULES

The AFL hopes improvements to the product can help lure wary supporters back and has unveiled new rules intended to speed up the game following years of complaints that defensive and congested contests are turning off fans.

Players who stand in front of opponents taking a free kick, known as "the man on the mark", will no longer be able to move from their mark, giving the player in possession more passing avenues.

Teams will also have to manage their match-day resources more carefully, with the number of player rotations permitted off the bench reduced to 75 from 90, meaning games could open up as players tire.

The AFL approved a late rule change on Wednesday allowing clubs to replace an injured or concussed player under a new medical substitute rule.

The 23rd player can only be activated after club doctors rule another player "medically unfit" to continue.

The rule has been slammed by the AFL players' union, whose boss Paul Marsh said teams would surely exploit it.

"The players are comfortable with (having) the concussion sub (substitute), but there is concern about how the broader injury sub will be gamed," he told Melbourne's Herald-Sun newspaper.

"It is impossible to see how it won’t."

(Reporting by Ian Ransom; Editing by Peter Rutherford)