Federer-Djokovic is one of the great rivalries of tennis, writes Greg Baum.
Semi-finals - implying play-off matches of equal gravitas - don't really cut it as a signpost for this stage of the 2016 Australian Open.
In another sport, they might be called major and minor semis, or heat and repechage. One features two players with a total of 27 major championships between them, the other at most Andy Murray's two. 
Thursday night's first semi could worthily be the final; it was at the past two majors.
Friday night's second semi - without knowing the combatants at the time of writing - could have been held over from the Kooyong Classic. Murray or no Murray, on the other side of the net will be a player ranked outside the top 10. That's way down in Bernie Tomic territory.
Thursday's match takes precedence chronologically, hierarchically and at the weigh-in. No tournament in this time is over until Novak Djokovic and Roger Federer have played one another.
By night's end, theirs will be the most prolific rivalry at major championships in the open era. In all tournaments, they have played 44 times, for 22 wins apiece. That's rivalry, as distinct from what Serena Williams and Maria Sharapova share, which is more like servitude.
Djokovic has also played Rafael Nadal 47 times and Andy Murray 30 times. Federer has played scores of matches against Nadal and Murray, too. But Federer also played eight matches against Andre Agassi and even one against Pete Sampras, at Wimbledon, and beat him. That was in 2001. In 2001 Djokovic was playing under-14s.
That is the fascination about their competition and Thursday night's match: Djokovic and Federer are peers, but not contemporaries. Djokovic is a dominant No.1 now, but so was Federer at Djokovic's age. When Djokovic is Federer's age, will he be in some new gang of four?
Contemplating Federer's historic standing, some critics mark him down because he has a losing record against Nadal and a break-even record against Djokovic. That implies that, say, Rod Laver was dominant against every opponent, at all times. He wasn't.
Federer's is great not just because he has always been a pleasure to watch, but because he was in the mix at 21 and he still is at 34. To some minds, he is improving again, defying gravity. One such mind is Djokovic's. He notes this new-old Federer is more intent on getting to the net, shortening rallies and games, messing with the prevailing defensive paradigm. To Djokovic, the great fidei defensor of baselines everywhere, the surveillance tapes must have been fascinating.
"He hasn't really dropped the level at all, I think, in the last seven, eight years that I've been playing against him on the top level," Djokovic said. "He's been playing always consistently well.
"I think his backhand is better than it was maybe five, six years ago. He's playing some terrific tennis on the hardcourt."
Federer admits going back to go forward (or is that advancing to return to a heyday?).
"I'm playing good tennis, fun tennis for me anyway," he said. "I really enjoy being able to come to the net more, like back in the day."
To most on the circuit, this is the day.
Djokovic, on the face of it, has changed nothing. Confounding insanity theory, he does the same thing over and over, for an ever-improving result, not least four wins in the past six majors.
Of course, he is making adjustments and refinements all the time. One this week was to skip a day's practice, not because his work ethic is wavering but because he is attuned to himself.
"Less is more sometimes," he said. "You need to recognise the moment. I've played a lot of tennis - maybe even too much. There was no concern for me that I would not feel the ball."
Intuitively, Djokovic should win this monster semi, by a margin. He is the better player now. His 2015 was titanic, Federer's merely excellent. But there remains a feeling in tennis's water that Federer has one more major left in him. If not at Wimbledon, here is as likely as anywhere. Hence, this finale before the final. Friday night: the postscript.


CLASH OF THE TITANS
Semi-final showdown between Novak Djokovic and Roger Federer Melbourne Park, Thursday, 7.30pm
HEAD TO HEAD
1-DJOKOVIC       22-22   3-FEDERER
Djokovic leads 8-6 at grand slams
28        Age   34     
Belgrade, Serbia      Born   Basel, Switzerland    
Monte Carlo, Monaco     Lives   Bottmingen, Switzerland   
188cm       Height  185cm     
80kg        Weight  85kg     
right-handed      Plays   right-handed    
1        Ranking  3     
$134 million      Prize money $139 million    
60        Career titles 88     
10        Grand slams 17     
(Australian Open 2008, 2011-2013, 2015;     (Australian Open 2004, 2006-07, 2010; French Open
Wimbledon 2011, 2014; US Open 2011, 2015)     2009; Wimbledon 2003-07, 2009, 2012; US Open
Australian Open win-loss record: 55-6     2004-2008) Australian Open win-loss record: 80-12