Why you should watch it
Because like so many so other games recently, as A-League fans have been blessed with meaningful clashes, there is an enormous amount riding on the result.
Wellington had a four-point lead at the top of the table ahead of this round beginning. Sydney was in third place, on the same number of points as both second-placed Melbourne Victory and fourth-placed Perth Glory and one point ahead of Adelaide United.
Under the current finals format, no team has gone on to win the grand final from outside of the top two at the end of the regular season, so, with just five rounds remaining and the top half of the table tightly packed, results from now on are critically important.
What happened last time they played
It was four days before Christmas and Sydney had the home-ground advantage. It looked the better side in a goal-less first half, but after the break it faded badly for no apparent reason and Phoenix began to find inroads.
Eventually, the chances were converted into goals, with Nathan Burns scoring in the 73rd and 81st minutes and Wellington winning 2-0.
Burns, the second-highest goal-scorer in the competition this season with 13, isn't playing on Sunday, but neither is Sydney's Mark Janko, who tops the goal-scoring list with 16. Both are away on international duty.
The Key battles
Wellington has proven its ability to cope without Burns and still win games. It has done that a couple of times already this season. But the added challenge here is that Michael McGlinchey is unavailable for Phoenix as well.
Phoenix coach Ernie Merrick likes to set up with a central striker and two wide attackers funnelling in, but with Tyler Boyd and Kenny Cunningham also out his options are limited.
The inclusion of ex-Western Sydney Wanderers player Kwabena Appiah in the squad suggests he'll get a run out wide.
Joel Griffiths and Roy Krishna in the other two attacking positions will still make life difficult for the Sydney FC defenders and that is where the most crucial battle in this game will take place.
Sydney's sometimes leaky defence is going to have play very tight on that pair.
Apart from that, Sydney has lost some serious attacking drive out of the midfield with Terry Antonis unavailable.
Phoenix has still got its most dominant force from that part of the field in Roly Bonevacia. The players directly ahead of Sydney's back four must work hard to try to shut him down.
Why Phoenix will win
The team is on a roll, having won four games straight. It has a structure and style that doesn't appear to be upset by changes, which is important in a week like this. It's strong in defence and Sydney is missing its most dominant attacker in Janko.
Why Sydney will win
It has done extraordinarily well on the road this season, with six wins and four draws from 10 games. That is much better than its home record. Shane Smeltz will get the chance to start a game with Janko out and he will relish the opportunity against one of his old clubs.
What the players are saying
Krishna says Phoenix won't panic in the absence of Burns and McGlinchey.
"It's a team sport and you can't rely on one player," he says. "It won't be a problem this week. We just have to believe in ourselves and enjoy it."
Sydney defender Matthew Jurman says that while the team can't simply expect its unbeaten away run to continue, it will be boosted by the knowledge that it has done so well on the road.
"It doesn't mean we're going to win the rest of our away games, but if we look at it one game at a time and concentrate hard on Sunday then we can get the three points," he said.
"Everybody needs to be confident that we can get the result."
What the coaches are saying
Depth was a problem for Wellington as recently as last season, but that has changed and Ernie Merrick is confident he can cover for a number of players being away on international duty.
"I've got really good options. This isn't going to be a patched-up team," Merrick said.
Sydney FC's Graham Arnold has tried to put the heat on the opposition by saying that leading the competition brings added pressure.
"Every game from now on is about pressure. It's about dealing with those situations," Arnold said. "When you've got something to hold on to, it makes you desperate to hold on to it.
"Sometimes you can play the game before you actually do play the game, whereas we've only got to worry about ourselves."
The villain
No club likes one of its former players to come back and do a job on it, so Smeltz will really rub the Phoenix faithful the wrong way if he knocks a goal or two in here and makes the difference.
Smeltz played the 2007-08 and '08-09 seasons with Wellington and scored 21 goals in 39 games. Ben Sigmund and Andrew Durante in the central defence for Phoenix will be well aware they can't allow him to get the ball in space.
The hero
Phoenix fans love Krishna - and why wouldn't they? He's a success story. Krishna couldn't break into an A-League squad in his early 20s, but he kept trying and made sure he improved and last season he finally got a chance at 26 with Wellington.
This season, Krishna is in very good company as the fifth-leading goal-scorer in the competition, with nine.
He won't be afraid of the added responsibility in attack with Burns and McGlinchey not there. He'll go looking for the ball and he'll have a crack at goal if he thinks it's on.
The match in numbers
These two teams have met 23 times overall and Sydney has a slight advantage, with 12 wins to Phoenix's 10.
There has only been one draw. But the last 10 clashes have seen Phoenix win six and Sydney four.
In Wellington, Phoenix has a clear advantage, having won six of the 10 clashes there and Sydney three, with one drawn.
That advantage has become even greater in recent times, with Wellington winning four of the last five clashes at home and Sydney taking the other.
The verdict
Antonis hasn't been starting much for Sydney recently, but he has made a big difference off the bench when needed and they could potentially miss him a lot in this game.
Wellington, if it doesn't take hold a of a game early, is wonderfully patient when it has to be and that was how it got Sydney last time.
The Phoenix midfield, led by the sublimely in-form Bonevacia, will create enough opportunities for Roy Krishna and Joel Griffiths to do something up front.
Sydney could get a draw if it keeps the opposition midfield under control, but that is going to be harder said than done in Wellington.
Phoenix to win.
