One of the great tennis rivalries resumes at the Australian Open on Thursday, once again posing the question whether ageing Roger Federer still has it in him to master Novak Djokovic.
The Serbian world number one, coming off an astonishing 2015 season which fell only one win short of a calendar-year Grand Slam, is zeroing in on a sixth Australian title in Melbourne.
Federer, at 34, is in the twilight of an imperious career including his all-time men's tennis high of 17 Grand Slam titles, and still has the game to remain in the men's top three.
Thursday's semi-final will be their 45th meeting in a head-to-head which is on a knife-edge at 22 wins each. Federer has lost their last three Grand Slam encounters, but still yearns for his elusive 18th major.
It will be 10-time major-winner Djokovic's 29th Grand Slam semi-final and sixth in Melbourne. His unbeaten Grand Slam run now extends to 19 matches and has not lost at a major tournament since he was defeated by Stan Wawrinka in last year's French Open final.
On the other hand, the Swiss legend's last major win was at Wimbledon in 2012, and since then Djokovic has accumulated five of his 10 Grand Slam titles, plus another five finals appearances.
"It's part of the reason why I'm still playing. I feel like I'm competitive at the top. I can beat all the guys on tour," said the Swiss, after trouncing Tomas Berdych in the quarter-finals.
"It's nice now that in the last three Slams I've been as consistent as I have been. I'm playing good tennis, fun tennis for me anyway.
"So I'm very pleased. It would mean a lot to me, no doubt about it."
Meanwhile, Djokovic said he was looking forward to the next instalment of his great rivalry with Roger Federer after he beat Kei Nishikori to set up an Australian Open semi-final with the Swiss.
"It's always a great challenge to play Roger," Djokovic said of Thursday's semi-final. "Any round feels like a final because of the fact that we are big rivals, we played so many times against each other. There's a lot of tension. There's a lot at stake. I'm expecting a great fight in two days."
Federer will be the sentimental favourite with the Australian crowd, who revere the Swiss for his past exploits on Rod Laver Arena where he has won four Australian crowns.
But Djokovic will be looking to step up after being taken to five draining sets by methodical Frenchman Gilles Simon, and only spasmodically producing his best against a misfiring Kei Nishikori in the quarters.
The world number one, with six-time slam champion Boris Becker in his camp, knows he has to produce his best tennis to beat arguably the greatest player of all time.
"Roger is playing really terrific tennis in the last two years. We played two Grand Slam finals last year," he said. "I know very well how good he plays, especially in the later stages of a major event.
"He always makes you play your best. My best is what is going to be necessary to win against him. Hopefully I'll be able to deliver."
Federer is constantly seeking ways to rejuvenate his game to stay among the elite and this year he added former world number three Ivan Ljubicic to his coaching team, joining long-time mentor Severin Luthi.
Djokovic is noticing the different emphasis in Federer's game under the new regime.
"It's obvious he's a very complete player. He's trying lately to come to the net more, kind of shorten up the rallies. He definitely has the game for that," the Serb observed.
"He's got a great variation from the backhand side with the short slice. He's got great defence, amazing offence. He's very complete.
"He puts constant pressure on the opponent. You have to be aware at all times.
"You've got to be tough. You've got to be concentrated. You have to be on the top of your game because 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."
The best of the best: Top five Federer-Djokovic clashes
2007 US Open final
- Djokovic was making his first appearance in a Grand Slam final and had beaten Federer in the Montreal Masters just weeks before. The Swiss legend was too strong for the 20-year-old Serb, winning in straight sets 7-6 (7/4), 7-6 (7/2), 6-4, but Djokovic led 6-5 in both the first two sets and held five set points in the opening set. Federer became the first man to win four successive US Opens since Bill Tilden 80 years earlier, but it was a warning for the Swiss.
2010 US Open semi-final
- Federer held two match points in the fifth set but couldn't convert them. The Serb held, broke Federer's serve, and then served it out to win 5-7, 6-1, 5-7, 6-2, 7-5. Djokovic denied Federer what would have been a seventh successive final appearance at Flushing Meadows.
2011 Australian Open semi-final
- The match lasted three hours even though it comprised only three sets. Djokovic took the first set in a tight tiebreaker. Federer raised his level to establish a 5-2 lead in the second set but was unable to serve out the set and Djokovic reeled off five straight games for a two-set lead. Federer rallied from an early break in the third set, but Djokovic won the final two games from 4-4 to claim a 7-6 (7/3), 7-5, 6-4 victory.
2011 French Open semi-final
- Super-confident Djokovic went into the tournament on the back of a 43-match winning run. Federer rolled back the years and won an exceptional 70-minute opening set in a tiebreak and then claimed the second set before the Serb took the third set and was serving for the fourth. The Swiss great fought back to take the match in a tiebreak closing out in four sets 7-6 (7/5), 6-3, 3-6, 7-6 (7/5) with his 18th ace at 9:37 pm local time.
2014 Wimbledon final
- The first Wimbledon final played between the two developed into an epic three-hour, 56-minute dogfight. They split two tie-breaks in the opening three sets before the Serb opened a 5-2 lead in the fourth set. Federer saved championship point as Djokovic dropped five straight games and the Swiss saved three break points at 3-4 but finally cracked in his next service game, as Djokovic won the title for a second time 6-7 (7/9), 6-4, 7-6 (7/4), 5-7, 6-4 to regain the world number one spot. He also prevented Federer from moving past Pete Sampras with a record eighth Wimbledon title.
Novak Djokovic v Roger Federer head-to-head record
2015 London Hard RR Federer 7-5, 6-2
2015 London Hard F Djokovic 6-4, 6-3
2015 US Open Hard F Djokovic 6-4, 5-7, 6-4, 6-4
2015 Cincinnati Hard F Federer 7-6 (7/1), 6-3
2015 Wimbledon Grass F Djokovic 7-6 (7/1), 6-7 (10/12), 6-4, 6-3
2015 Rome Clay F Djokovic 6-4, 6-3
2015 Indian Wells Hard F Djokovic 6-3, 6-7 (5/7), 6-2
2015 Dubai Hard F Federer 6-3, 7-5
2014 Shanghai Hard SF Federer 6-4, 6-4
2014 Wimbledon Grass F Djokovic 6-7 (7/9), 6-4, 7-6 (7/4), 5-7, 6-4
2014 Monte Carlo Clay SF Federer 7-5, 6-2
2014 Indian Wells Hard F Djokovic 3-6, 6-3, 7-6 (7/3)
2014 Dubai Hard SF Federer 3-6, 6-3, 6-2
2013 London Hard RR Djokovic 6-4, 6-7 (2/7), 6-2
2013 Paris Hard SF Djokovic 4-6, 6-3, 6-2
2012 London Hard F Djokovic 7-6 (8/6), 7-5
2012 Cincinnati Hard F Federer 6-0, 7-6 (9/7)
2012 Wimbledon Grass SF Federer 6-3, 3-6, 6-4, 6-3
2012 French Open Clay SF Djokovic 6-4, 7-5, 6-3
2012 Rome Clay SF Djokovic 6-2, 7-6 (7/4)
2011 US Open Hard SF Djokovic 6-7 (7/9), 4-6, 6-3, 6-2, 7-5
2011 French Open Clay SF Federer 7-6 (7/5), 6-3, 3-6, 7-6 (7/5)
2011 Indian Wells Hard SF Djokovic 6-3, 3-6, 6-2
2011 Dubai Hard F Djokovic 6-3, 6-3
2011 Australian Open Hard SF Djokovic 7-6 (7/3), 7-5, 6-4
2010 London Hard SF Federer 6-1, 6-4
2010 Basel Hard F Federer 6-4, 3-6, 6-1
2010 Shanghai Hard SF Federer 7-5, 6-4
2010 US Open Hard SF Djokovic 5-7, 6-1, 5-7, 6-2, 7-5
2010 Toronto Hard SF Federer 6-1, 3-6, 7-5
2009 Basel Hard F Djokovic 6-4, 4-6, 6-2
2009 US Open Hard SF Federer 7-6 (7/3), 7-5, 7-5
2009 Cincinnati Hard F Federer 6-1, 7-5
2009 Rome Clay SF Djokovic 4-6, 6-3, 6-3
2009 Miami Hard SF Djokovic 3-6, 6-2, 6-3
2008 US Open Hard SF Federer 6-3, 5-7, 7-5, 6-2
2008 Monte Carlo Clay SF Federer 6-3, 3-2 - retired
2008 Australian Open Hard SF Djokovic 7-5, 6-3, 7-6 (7/5)
2007 US Open Hard F Federer 7-6 (7/4), 7-6 (7/2), 6-4
2007 Montreal Hard F Djokovic 7-6 (7/2), 2-6, 7-6 (7/2)
2007 Dubai Hard QF Federer 6-3, 6-7 (6/8), 6-3
2007 Australian Open Hard R16 Federer 6-2, 7-5, 6-3
2006 Davis Cup SUI v SRB Hard RR Federer 6-3, 6-2, 6-3
2006 Monte Carlo Clay R64 Federer 6-3, 2-6, 6-3
With inputs from AFP