MADRID: Rafael Nadal will worry as much about his problem knees as he will about his impeccable tennis when he faces old rival Roger Federer for the 20th time in the final of the Madrid Masters on Sunday.

The Spanish king of clay made another agonising and heroic stand, saving three match points in a four-hour semi-final marathon to beat gallant Novak Djokovic 3-6, 7-6 (7/5), 7-6 (11/9).

The epic clash, which stretched deep into the evening, preceded Federer's silky smooth 6-3, 6-4 win over Argentine Juan Martin Del Potro, which finished in less time than one of Nadal's sets.

World number one Nadal, the winner of his last 33 claycourt contests, stretching back a year, confessed that his troublesome knees are a concern as he bids for his fourth straight clay title this spring.

"I had problems in the first set and some of the second. It was difficult for me to play at 100 percent. I had trouble pushing off," said the top seed who needed his right leg to be heavily strapped just above the knee in the second set.

"They were not responding so well but I got an anti-inflammatory and some hot cream on court and that helped. It's tough but I hope I can play normal."

Nadal stands a formidable 13-6 over Federer, winning five of their last six meetings including the Grand Slam finals in Paris, Wimbledon and Melbourne.

Federer dominated Del Potro in a tidy win in stark contrast to the Nadal-Djokovic rollercoaster.

The 13-time Grand Slam winner is still seeking his first title of the season, but is looking forward to the final.

"Playing Nadal anywhere is a challenge, especially on clay. The extra factor here is that this is in Spain," said the Swiss.

"This has never happened before. I'm excited to play him. We've not played so often (this year), the Australian Open was the last time. Now we are on clay, his surface. We'll see what happens."

Nadal came through on his second match point to claim his third win in a month over the third-seeded Djokovic at the Masters level after the Monte Carlo and Rome finals.

Nadal, who has not been beaten on clay since May, 2008, in Rome, wrapped up the semi-final on his second match point to leave Djokovic physically and mentally shattered.

"It's happened too often," said the Serbian world number four of his recent record against the Spaniard.

"It's very disappointing to play as well as I have and still lose the match. I played one of my best matches ever," added Djokovic after striking 37 winners in the enthralling struggle.

"I was a couple of points from the victory. I even played a few points above my limits and I still didn't win."

Djokovic, who would have reclaimed his world number three spot from Andy Murray, which he lost only last week, had he reached the final added: "I hope I'll be ready for the French Open."

Djokovic showed he meant business after his recent losses to Nadal, earning the first break of the match in the second game off a double-fault.

The Serbian third seed kept up the attack on the world number one, reaching 4-1 on his way to the early lead.

But after losing his first set of the tournament, Nadal was in no mood for more home disappointment, saving two break points in the ninth game of the second for a 5-4 lead, eventually squaring the match with a tiebrekaer.

The third set only lifted the tension as Djokovic broke in the fourth game only to hand the advantage straight back in the fifth.

The final-set tiebreaker provided all the drama possible for the sellout crowd of 15,000 as both men grappled for the win in the last tune-up before Roland Garros which starts on May 24.

Nadal has now reached his fourth claycourt final of the spring and took his career record over Djokovic to 14-4.

The Spaniard improved to 19-0 on clay in 2009 and 150-4 since 2005.

He lost his only clay semi-final out of 28 played back in 2003 as a teenager at Umag in Croatia.