Johannesburg - Imran Tahir broke a menacing partnership between Ricky Ponting and Usman Khawaja with what turned out to be the second last ball of the day at the Wanderers Stadium on Sunday to leave the second Test balanced on a knife's edge.Scorecard after Day 4The Pakistan-born spinner dismissed the Pakistan-born batsman with a perfectly pitched googly that found the outside edge of the left-hander’s bat and gave Jacques Kallis his second slip catch of the innings.It brought to an end a third-wicket partnership of 122 runs in 34 overs with Khawaja’s contribution being 65 off 110 balls with 10 fours and a six. It was his maiden half-century in Test cricket and could not have come at a better time for his side.It followed a double breakthrough by Vernon Philander who dismissed both the Australian opening batsmen in his new ball spell.Ricky Ponting finished the day unbeaten on 54 (104 balls, 6 fours) with the Australians needing a further 168 runs on Monday’s final day.It is impossible to say which team is on top at the moment. The Proteas will feel they are no more than a couple of wickets away from a major breakthrough into the tail while the Australians know that another century partnership will finish the job for them.Imran is probably going to be the key bowler as even Ponting failed to pick his googly on one occasion.The Australians had earlier dismissed the Proteas for 339 to leave themselves with the victory target of 310 to square the two-Test series. New Cap Patrick Cummins completed an outstanding debut with innings figures of 6/79. At one stage he was on a hat-trick immediately after lunch when he dismissed Vernon Philander and Morne Morkel with the first two balls of the afternoon session.Hashim Amla completed his 14th Test century - this puts him joint fourth on the South African all-time list along with Daryll Cullinan and Herschelle Gibbs - early in the day but the Proteas were set back by the quick dismissal of AB de Villiers, Ashwell Prince (unnecessarily run out), and Amla in quick succession.It needed a 48-run stand between Philander and Dale Steyn to stabilise the innings and take the target beyond 300. In the end Steyn made the third highest score of the innings of 41.