Wimbledon had been waiting for Novak Djokovic to have an off day.

Given the capacity with which the sport's elite have lapsed into fits of vulnerability already this summer, it had been assumed that the world No.1 was bound to stumble at some point, with the Serb stating earlier this week that Tommy Haas would offer a "big challenge". Djokovic faced him yesterday and duly won 6-1, 6-4, 7-6 (7/4) to reach the quarter-finals.

It was about as close to a bad day at the office as Djokovic is likely to permit; winning his first set after just 25 minutes but completing the match at the more slovenly pace of two hours and 12 minutes. His rhythm was interrupted by a break in his serve during both the second and third sets, with Haas causing his celebrated opponent mild agitation on a day which has been dubbed "Manic Monday" by organisers.

In the women's draw, Serena Williams had earlier been humbled on Centre Court but Djokovic would only stumble. He refused to fall. There has been a temptation to consider that all players are fallible since Roger Federer and Rafael Nadal have already been sent home but Djokovic was only caught by Haas, the 26-year-old never seeming likely to haemorrhage points against the German. He even took the tie-break in the third set 7/4.

"It was never going to be easy, I knew he would improve," said Djokovic, who will face Tomas Berdych in the last eight. "He missed a lot of easy shots in the first. I had to work and I was serving well, returning exceptionally [well] in important moments. It was a minor setback at 5-3, I was 30-0 and made a couple of double faults but I am glad I closed it out."

There was a similar sense of relief for Berdych yesterday, who became caught up in an untidy match against the unseeded Bernard Tomic which ended 7-6(4), 6-7(5), 6-4, 6-4. The Czech has already upset the world No.1 at Wimbledon – knocking him out in the semi-finals three years ago, before losing to Nadal in the finals – although he seemed philosophical about their rematch this year. "It's a nice challenge," said Berdych. "I know how to beat him, let's try for the same."

The mental strain of trying to find a way past a player as domineering as Djokovic will be telling, although the physical strain being placed on Juan Martin del Potro appears more clearly defined. The Argentine limped into his press conference following a 6-4, 7-6(2), 6-3 victory over Italy's Andreas Seppi and will now have just two days to recover until taking on David Ferrer.

Del Potro's movement looked tentative and at times slightly cumbersome, even if his big hitting ensured he had few problems against Seppi. "The ankle is good and the knee is not good," he said afterwards. "The doctor says it's nothing too dangerous and that's positive. But it bothers me a lot.

"I couldn't extend the knee 100%. And the tapes for today help me to be careful in some movements. But it's not enough. But I have a day and a half until my next match and I will do all the treatments to see if I can be better."

He disappeared to take an ice bath shortly afterwards, but he is unlikely to get any mercy from fourth seed Ferrer – also suffering with an ankle injury – who secured a 6-7 (3/7), 7-6 (8/6), 6-1, 6-1 win over Croatia's Ivan Dodig. "I think he [will be] No.3 in the world after this tournament. He deserves to be there," Del Potro said. "He is a good player on grass. He is a very good player on all surfaces."