The Bristol player led 5-3 going into the second session then moved to 7-4 before his Chinese opponent won four frames in succession.
Trump, who beat world champions Neil Robertson and Graeme Dott en route to the last four, said during the interval that he was not happy with his form.
"Not feeling great out there need to get my game face on!," he tweeted.
The 21-year-old's performance in Friday's early session was in stark contrast to his blistering effort on Thursday, which included a magnificent second-frame 110 decorated with audacious shots.
Ding began the second session well with a break of 91 - the key shot, a glorious blue into the left centre pocket with the cueball screwed back to open the pack of reds.
The 24-year-old world number four then led 56-0 in the 10th frame before inexplicably leaving a pink over the pocket. Trump took advantage following a safety exchange to re-establish a two-frame lead.
He then took the next before Ding began his fightback. The two-time UK champion appeared to waste his chance to win the 12th framewhen he faltered after racing into a 43-point lead, but Trump failed to punish him.Trump pot shots put Ding on back foot on Thursday
Trump looked set to steal the next after Ding had established a sizeable lead. But after fluking the blue and potting the pink, China Open winner Trump faced a difficult black into the left centre pocket. He missed the pot and Ding eventually won the 14th frame too.
The Chinese player then took a scrappy last two frames to take a 9-7 lead into the Friday evening session.
Neither player has previously played in a Crucible semi-final, with Trump beating former champions Neil Robertson and Graeme Dott to get there.
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