At 5-3 in the final set of a match that has brought the women’s championship of the US Open to vibrant life, Kim Clijsters urged herself on with a “C’mon” every bit as guttural and frightening as those trademarks of her one-time fiancé, Lleyton Hewitt.

The magic worked. Not only is Clijsters, who is playing only the third tournament of her comeback after quitting the tour in 2007 to start a family, into the quarter-finals of the only grand-slam tournament she has won, but in doing so she put Venus Williams to the sword in an hour and 42 minutes. There were so many extraordinary fluctuations that Mary Carillo, in the CBS TV booth, was moved to describe it as “whack-a-doo” tennis.

Clijsters beat Venus 6-0, 0-6, 6-4, which is certainly a whack-a-doo scoreline. First the Belgian thumped the ball hard and inside the lines, then Williams took over in the same vein, before Clijsters did it again. The rallies were brutal and bruising, but the close-quarters tennis left much to be desired. For all her five Wimbledon singles titles — not forgetting the doubles crowns as well — there is something pretty horrid about Williams’s backhand volley.

Still, such matters are not high on Clijsters’s mind. She is within three victories of claiming the title, something that she would have dismissed as an absurdity when the call came from the All England Club in March asking if she might consider taking part in an exhibition to launch the Centre Court roof at Wimbledon. She got herself into shape for that festivity in May, realised she was striking the ball with all her old abandon, stepped up the gym work and the practice-court time and chose to take on the Sony Ericsson WTA Tour again.

Clijsters, 26, has been moved by the level of pleasure generated by her return and there were times during the final set when the Americans seemed more voluble for her than they were for Williams.

There are other examples of players who have come back to tennis having had a baby — Lindsay Davenport returned in August 2007, won two of her first three tournaments and retired for a second time a year later — but none who threatened in a grand-slam event within weeks. She is striking the ball as if she has never been away. “It was such a weird match,” Clijsters said. “When I lost the second set 6-0, I thought, ‘Let’s just start over again.’ ” Next, in the last eight, is Li Na, of China.

Rafael Nadal needs to win the US Open to become the seventh male to complete the set of grand-slam tournaments, but it surely cannot happen this year. Though we know the extraordinary depths to which this young Spaniard can dig, he would need the use of one of the drills that seem to be excavating the corner of every block here to get the job done.

The constant speculation as to the state of the 23-year-old’s knees is one thing, but there was a moment during the third set of his third-round match yesterday that gave further cause for concern. Having ointment and a bandage applied to his navel region is nothing like surgery, although the thought that he may be susceptible to stomach-muscle injury is not a comforting one. Nadal came through against Nicolás Almagro, his fellow Spaniard, 7-5, 6-4, 6-4 and said that he did not want to talk about injuries.

Andy Roddick, who had been hoping that this event would offer succour after a crushing Wimbledon final defeat by Roger Federer, discovered that John Isner is now living up to every inch of his 6ft 9in frame. When he saved a match point in the fourth set with an ace, the assumption was that Roddick would clinch the match in the fifth. But Isner, 24, pumping down 38 aces but mixing that with greater flexibility of movement than his stork-like legs should allow, would not be denied.

Whether Isner can recover from this to stare down Fernando Verdasco, the Spaniard, in the fourth round must be doubted. But all of Georgia — the state from which Melanie Oudin, the 17-year-old upstart package of the women’s event also comes — will be rooting for him.