When most fans think of big Hyundai A-League rivalries they think of the Sydney or Melbourne clashes, the Mariners-Jets F3 derby, or the Sydney FC-Melbourne Victory Big Blue.
However, followers of the A-League from its beginnings in 2005 will recall the most classic encounters of the first season were between Adelaide United and Sydney FC.
Their battles throughout 2005/06 were intense confrontations that have rarely failed to disappoint since.
When the A-League kicked off in 2005 it was Sydney FC who were supposed to rule the roost from day one.
Their high-quality and flamboyant squad, with overseas stars on and off the field, were early favourites to take out the inaugural A-League Premiership trophy.
However it was the Reds from Adelaide, reliant on gritty home-grown characters like coach John Kosmina and captain Ross Aloisi who took the first home-and-away season by storm.
Adelaide United smashed their way through the opening A-League season, taking out first position on the final ladder well clear of second-placed Sydney FC.
At one stage the Reds were 11 points ahead at the top.
Ultimately however the Sky Blues had the last laugh in 2005/06, beating Adelaide in the major semi final over two legs.
A demoralised United could not win the preliminary final the following week and so missed out completely on the grand final, which was taken out by Sydney.
And so, in an instant, Adelaide United's regular season dominance was all but forgotten.
Stars of the teams in season one were Fernando Rech, Shengqing Qu and Carl Veart for the Reds, with Dwight Yorke, Steve Corica and David Carney doing good work for Sydney.
And Japanese luminary “King” Kazu Miura scored both Sydney goals in its 2-1 mid-season win over Adelaide.
Another legendary name in Benito Carbone was on the scoresheet in Sydney's 4-1 victory over Adelaide in 2006/07, and over the following few years South American stars Rech, Alemao, Cassio and Cristiano all scored for the Reds in matches against their Sydney rivals.
After another Championship-winning season for Sydney in 2010/11, the Adelaide-Sydney head-to-head record was tipping Sydney's way, with nine wins overall to United's six.
However a clean sweep of three wins by Adelaide in 2010/11, the first time a reigning champion A-League team had suffered such a clean sweep, brought the teams' record to a level nine wins each.
Adelaide's resurgence in their rivalry with the Sky Blues started with a 3-1 win in early 2010/11, Sergio Van Dijk (2 goals) and Mathew Leckie the scorers for United.
Since then the head-to-head record has turned in Adelaide's favour, now ahead 16 A-League wins to 13 leading into their Round 18 clash this season.
Adelaide's goals for and against record in the 34 games stands at 53-46, meaning the next goal will be the 100th goal scored in Adelaide United-Sydney FC A-League matches.
The two sides also met in the first two Westfield FFA Cup editions, the South Australians winning the quarter final encounter in 2014 and Round of 16 match last year.
And while these two FFA Cup wins puts Adelaide even further in front on the head-to-head count 18 wins to 13, Sydney FC have good claim to winning the most meaningful of these matches having knocked Adelaide out of A-League finals races two times out of two, in 2006 and 2015.
In looking for factors to indicate the outcome of their Round 18 match, it's hard to ignore Adelaide United's form run against all teams in 2015/16.
The Reds are unbeaten in their past nine games, maintaining a turnaround that saw them not winning any of their previous eight matches this season.
Adelaide United's current form turnaround is truly of historic proportions.
The A-League ladder at the start of February however shows Sydney ahead of Adelaide. Losses in Rounds 16 (against the Victory) and 17 (Brisbane) will only make Sydney's talented and proud squad hungrier for their Round 18 clash against the Reds.
It should be another cracking confrontation at Coopers on Friday night.
Follow Andrew Howe’s Australian football stats updates on Twitter @AndyHowe_statto
This article was originally published at: http://www.a-league.com.au/article/adelaide-united-v-sydney-fc-hyundai-a-leagues-original-rivalry/hvex6acadipg1fsvlpy0544fs.