LONDON (AFP) - Novak Djokovic and Andy Murray edged closer to an epic Wimbledon final showdown on Monday while Poland played Grand Slam gatecrashers by guaranteeing at least a semi-final spot.

World number one Djokovic, the 2011 champion, dropped his serve for the first time at this year's tournament before beating Tommy Haas, the 35-year-old German 13th seed, 6-1, 6-4, 7-6 (7/4) in the fourth round.

Djokovic will be playing in his 17th successive Grand Slam quarter-final where he will face Tomas Berdych, the seventh seed and 2010 runner-up, who beat Australia's Bernard Tomic, 7-6 (7/4), 6-7 (5/7), 6-4, 6-4.

Djokovic boasts a 13-2 career record over Berdych but lost to him at Wimbledon in 2010.

"Haas is always a tough opponent. I had lost to him twice on grass before. He has great variety in his game and I always knew it would be a tough challenge," said Djokovic, who fired 13 aces and 40 winners.

"It was never going to be easy. I had to work for my games but I served well and returned well. I am happy to have closed it out in straight sets.

"I am playing really good tennis now, maybe even better than when I won the title in 2011." Second seed Murray took another step closer to ending Britain's agonising 77-year wait for a men's champion when he survived a second set wobble to carve out a 6-4, 7-6 (7/5), 6-1 win over Russian 20th seed Mikhail Youzhny.

Murray, the runner-up to Roger Federer in 2012, will face Spain's Fernando Verdasco for a place in the semi-finals.

The Scot has an 8-1 winning record over unseeded Verdasco, a former top-10 player.

Murray had to battle back from 2-5 down in the second set to quell the threat posed by Youzhny, who needed treatment on a shoulder injury after one game of the third set.

Murray put in an impressive serving performance, firing 15 aces and hitting 45 winners past the experienced Russian.

"It was a tough match, the first couple of sets especially. He also had a few chances at the beginning of the third set as well but once I got ahead in the third I concentrated very hard not to let him back in," said Murray.

The Scot said he wasn't getting carried away by the national hysteria as he tries to become Britain's first champion since Fred Perry in 1936.

He is wary of becoming another shock Wimbledon victim, like Roger Federer and Rafael Nadal.

"Roger's lost and Rafa's lost. All these guys are better players than me and have achieved a lot more than me and so if they can lose so can I. I just concentrate on the next one and try and get through it," he said.

Verdasco, a former world number nine who now stands at 54 in the world and is playing his 41st successive major, reached his first Wimbledon quarter-final with a 6-4, 6-4, 6-4 win over unseeded Frenchman Kenny De Schepper.

Davis Cup teammates Jerzy Janowicz and Lukasz Kubot set-up the first ever all-Polish quarter-final.

Janowicz, the 24th seed, defeated Austria's Jurgen Melzer, 3-6, 7-6 (7/1), 6-4, 4-6, 6-4 on the back of 16 aces and 34 winners as he reached his first Grand Slam quarter-final.

The 22-year-old was joined in the last eight just moments later by 31-year-old Kubot, the lowest-ranked player left at 130 in the world, who defeated France's 111th-ranked Adrian Mannarino, 4-6, 6-3, 3-6, 6-3, 6-4.

The last Pole to reach the last-eight at Wimbledon was Wojtek Fibak in 1980.

"It's unbelievable what is going on right now. We have two players in the quarter-finals and a woman in the quarter-finals. I think this is by far the best that possibly could happen to Polish tennis," said Janowicz.

Spanish fourth seed David Ferrer reached the quarter-finals for the second successive year by firing 53 winners in his 6-7 (6/8), 7-6 (7/3), 6-1, 6-1 win over Croatia's unseeded Ivan Dodig.

He goes on to tackle Argentine eighth seed Juan Martin del Potro who reached his first Wimbledon quarter-final with a 6-4, 7-6 (7/2), 6-3 win over Andreas Seppi, the 23rd seeded Italian.

Del Potro, the 2009 US Open champion, admitted that he is still being bothered by knee trouble.

"It bothers me a lot. I couldn't extend 100 percent the knee. The tapes for today helped me to be careful in some movements. But it is not enough," said Del Potro.

The 31-year-old Ferrer, who was defeated in the French Open final by Nadal last month, will be playing in his seventh consecutive Grand Slam quarter-final.