THE big four, already shorn of Andy Murray at the 2013 French Open, are not having it all their own way. Rafael Nadal has dropped two sets already, Roger Federer was pushed to the brink by Gilles Simon, and then Monday Novak Djokovic found himself a set adrift in the early going against Philipp Kohlschreiber.

Normal service was duly resumed, as it was for Rafa and Roger, but the chasing pack certainly seems to be, if not getting closer to the leading quartet, then at least chipping slowly away at the edifice that the foursome has built up over the past three years of Grand Slam invincibility.

It should perhaps have come as no surprise that “Kohli” won the opener. The only previous time that the two had met on clay was here four years ago, and the German won in straight sets, 6-4, 6-4, 6-4. An early break gave the 29-year-old the advantage, and he duly served out for a fourth straight 6-4.

That, however, was as good as it got for the crisp-hitting No.16 seed, who had recently made the final on the Munich clay only to lose to fellow German (and indeed fellow fourth-rounder here) Tommy Haas. Djokovic shook off his early sloth and dominated both in offence and defence, picking holes in Kohlschreiber’s game by attacking always at just the right moment.

There was a still a nervous moment at the end of the match, when Nole turned out to need two bites at the cherry to serve out for victory. 5-2 and cruising became 5-4 and wobbling, but Djokovic showed just why he is the world number one and concluded the match 4-6, 6-3, 6-4, 6-4.

His dream of taking the only Slam to elude him to date is still alive, with another German – the above-mentioned veteran Haas – standing between him and a place in the semi-finals, where a certain Majorcan could be lying in wait.

“Conditions were difficult for both of us but he adjusted better,” conceded Djokovic. “He was more aggressive. I was still trying to find my rhythm on the court and the set was gone. I needed to earn my victory.”

One day, the cracks will get bigger, but though the rest of the ATP tour still chips away at the edifice, the big four continue to defend their impregnable fortress.