All eyes were on Jennings at the start of England's second dig, the opener in need of a substantial contribution after a first-innings duck continued his run of low scores.
Jennings, who'd nudged two balls off the inside edge passed the stumps and was dropped on six, all off Philander's bowling, was not out on 34, while Westley, who looks a very composed no.3 was on 28.
Vernon Philander  was the last man left standing for the Proteas despite having spent the night in hospital on an intravenous drip after a suspected viral infection worsened.
But third slip Dean Elgar could not hold the sharp chance and the ball went for four.
Going into day three, the South Africans are hanging by a thread at 126 runs for eight, some 28 runs away from avoiding the follow-on.More news: Greece announces new 5-year bond issue after 3-year market exile
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Keaton Jennings, having been fortunate to see of Vernon Philander's opening spell, along with debutant Tom Westley put England in a powerful position before the rain saw play halted midway through the second session here on Saturday.
Bavuma followed up by driving Roland-Jones for a boundary through mid-on.
But a valuable ninth-wicket partnership of 47 ended when Morkel (17) nicked Anderson to Cook at first slip. The four-match series between England and South Africa is so far level at 1-1 with England winning the Lord's Test to begin Joe Root's captaincy career in the most ideal manner after the right-hander celebrated his new role with a flamboyant 190 but they lost the plot in the second encounter and stood on the losing side at Trent Bridge.