Novak Djokovic came through his toughest test at Wimbledon so far, beating the hard-hitting Tomas Berdych in straight sets to make his 13th consecutive grand slam semi-final.

Djokovic wasted four break points in the first set and was broken twice in the second, but he ultimately came through to record a deserved 7-6 (7/5) 6-4 6-3 win against the seventh seed.

The top seed, who is yet to drop a set in the tournament, will play Juan Martin Del Potro in the last four and is still on course to play Andy Murray in the final.

Djokovic admitted afterwards that he thought he could have lost the match.

The 2011 champion told the BBC: “It was a strong start for me today. I had a few break points, a few chances to break in the first set, but I didn’t manage to do that and the tie break was decided by one mistake with a forehand at 6-5.

“It was a very close match, it could have gone either way. He could have won the first two sets, he had a double break in the second.

“I don’t know how I managed to go ahead, I don’t know how I turned it around. I am really happy with the performance. I am playing some of the best tennis on grass of my career.”

Djokovic was glad to see fellow Serb and Manchester United captain Nemanja Vidic in the crowd, along with former Chelsea striker Andriy Shevchenko, who had also come to watch.

“It really means a lot to see Nemanja with Andriy here,” Djokovic said. “I have only met (Vidic) once before briefly, but we are glad that he accepted the invitation to come.”

Argentine Juan Martin del Potro recovered from a horrible early fall to overpower David Ferrer 6-2 6-4 7-6(7-5) and reach the Wimbledon semi-finals for the first time.

The towering eighth seed said he had been on the verge of pulling out when he injured his already-bandaged left knee after skidding on the fifth point of the match and crashing down in a heap.

But after lengthy treatment and a pain-killing pill the 24-year-old former U.S. Open champion returned to the court to produce one of the best displays of his career. Using his condor-like wingspan to make up for any lack of movement, del Potro unfolded his right arm to deliver a barrage of mighty serves and thumping forehands.

His forehand was unplayable at times and it was fitting that he finally broke the dogged resistance of fourth seed Ferrer with his 22nd clean winner from that flank. “It was the same thing as my third-round match,” del Potro, who injured his knee when falling on Saturday against Grega Zemlja, told the BBC.

“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. “But I played my best tennis in this tournament so far and I’m so happy to be in the semis.”