Can underdog Roger Federer upset iron man Novak Djokovic in Wimbledon semi-final?

In-form Swiss must defeat World No 1 to reach final as pair meet for first time on surface

Novak Djokovic: World No1 said Roger Federer is 'probably the best player in history' (PA)

Roger Federer has won a record 16 Grand Slam singles titles, including six at the All England Club, but when he faces Novak Djokovic in the semi-finals this afternoon the 30-year-old Swiss will be in the unaccustomed position of underdog.

There was a time when it would have been unthinkable for the former world No 1 to be second-favourite going into any match on grass, but Djokovic's form over the last 18 months, coupled with Federer's failure to win a Grand Slam title for two and a half years, have turned the tables. Djokovic, the defending champion here, has won four of the last six Grand Slam tournaments, while Federer has reached only one final – when he lost to Rafael Nadal at last year's French Open – in his last nine appearances at the majors.

The two men are familiar foes. Remarkably, Federer and Djokovic have been in the same half of the draw in 15 of the last 17 Grand Slam events. They have played each other 26 times, yet this will be their first meeting on grass.

Halle in Germany is the only grass-court tournament other than Wimbledon that Federer has played in recent years and he missed it on the only occasion when Djokovic played there, winning the title in 2009. Because of their rankings, Federer and Djokovic only meet these days in semi-finals or finals (their last 22 meetings have all been in semi-finals or finals). This is the first year when both men have reached the last four here.

While Federer has been the ultimate grass-court player for most of the last decade, Djokovic has grown to love the surface. The slowing-down of the courts in recent years plays into the hands of players like Djokovic who are at their most comfortable rallying from the baseline, though the Serb is much more confident at the net than he was two years ago.

Djokovic admits that in his early years Federer and Rafael Nadal had a psychological hold over him, but that has changed in the last year and a half. Djokovic has won six of his last seven meetings with Federer – his only defeat came in the semi-finals at last year's French Open when the Swiss ended the Serb's 43-match winning run – and won in straight sets in their most recent encounter, last month in the semi-finals at Roland Garros.

Federer remains as formidable as ever in the early rounds of Grand Slam tournaments – he has reached the quarter-finals or better of every event since the 2004 French Open – but in the last two years he has fallen short at the latter stages.

Djokovic has become the game's iron man. Physically, he has proved himself up to every challenge, as when beating Andy Murray and Nadal in successive matches lasting nearly five and six hours to win the Australian Open this year. Mentally, nobody plays the big points better. He saved four match points before beating Jo-Wilfried Tsonga at the 2012 French Open and in the US Open semi-finals of 2010 and 2011 he beat Federer in five sets after saving two match points.

Federer, who is aiming to become the first man to reach eight Wimbledon finals and would reclaim the world No 1 ranking by winning the title, believes Djokovic's consistency has made the difference.

"He used to maybe have a match where he wouldn't play so well at times or lose early in a tournament for some reason," Federer said. "He also had some health issues early on with his breathing, I thought. Little things like that obviously play a role in the everyday grind on tour. I think he's been able to put a lot of these things aside and he seems a very complete and happy player out there right now."

Federer is pleased with his own form. "I've been playing well for a year now," he said. "I'm happy that going ahead into the semis I'm not tired, I'm not injured, I'm not anything. I'm fresh and ready to go. That's how I want to feel before a semi-final. It's been a better tournament for me than in Paris, where I struggled all the way through. Here I think I've had some great matches."

Djokovic described Federer as "probably the best player in history" and added: "Most of our matches that we play against each other are very close. Very few points decide the winner."