LONDON, July 3 (Reuters) - An imperious Novak Djokovic proved unstoppable and courageous Juan Martin Del Potro indomitable as both claimed straight-sets victories on Wednesday to set up a Wimbledon semi-final clash.
Top seed Djokovic reeled off his fifth successive three-set victory although his quarter-final against Czech seventh seed Tomas Berdych was harder than the 7-6(5) 6-4 6-3 scoreline suggested.
Untroubled so far as he targets a second Wimbledon title, the clinical Serb had to find his best tennis to subdue Berdych who beat Djokovic in their only previous grasscourt meeting.
"It was a very close match, and that's what I expected," Djokovic, who lost to Del Potro in an Olympic bronze-medal match at Wimbledon last year, told a news conference.
"Tomas is a very powerful player. We went toe-to-toe in the first set."
Del Potro looked down and out after five points against Ferrer when his already-bandaged left knee crumpled as he tumbled while chasing a wide ball behind the baseline.
Grimacing in pain the six foot six eighth seed climbed up from the turf, however, and after treatment and a pain-killing tablet produced a sledgehammer-like performance to beat fourth seed David Ferrer 6-2 6-4 7-6(5).
Such was the venom in his murderous forehand that any injury concerns were put aside as he struck 22 winners on that flank alone, including a screamer on match point.
"It was the same thing as my third-round match," said Del Potro, who first injured his knee when falling on Saturday against Grega Zemlja.
"I was very close (to pulling out) because I felt a lot of pain in the beginning of the match, it was exactly the same like I did before. I twisted my knee once again and the doctor gave me some magic pills and I could finish the match."
Del Potro's medical team will work overtime to patch him up for a semi-final against world No.1 Novak Djokovic on Friday.
"I think it's going to be dangerous if I'm not careful in the next few days," he said.
The men's quarter-finals, featuring five of the eight tops seeds despite a tournament full of shocks, conclude later when home favourite Andy Murray faces unseeded Spaniard Fernando Verdasco and Lukasz Kubot faces Jerzy Janowicz in the first all-Polish grand slam quarter-final.
The winner on Court One will become the first Polish man to reach the semi-final at a slam and join compatriot Agnieszka Radwanska who faces Sabine Lisicki in the women's semi-finals on Thursday.
As well as those with Centre Court tickets, thousands of people were already positioned on the grassy slope opposite a giant video screen in the heart of the All England Club to cheer on Murray as he continues his bid to become Britain's first Wimbledon men's singles champion since 1936.
To do that, barring another humdinger of a performance by Del Potro, Murray will probably have to beat Djokovic.
The 26-year-old has looked in almost total control so far and even when the dangerous Berdych began to threaten, Djokovic found an extra gear to reach his 13th consecutive grand slam semi-final.
Djokovic trailed by two breaks in the second set but quickly snapped back into action to surge to victory.
"You have to play really on the limit, playing really great to have a chance to beat him," Berdych said.
"That was not my case today."
Editing by Ed Osmond