The world number one, who has held top spot since his title run 12 months ago, is attempting to avoid completing a year without a trophy for the first time since 2013.
Trump, now back with the cue that carried him to the 2019 World Championship, looked sharp in patches.
He produced a 117 in the third frame and stole the fourth with a composed clearance of 65 after trailing heavily.
A long 37-minute fifth frame pushed him 4-1 ahead, but Si, who beat Trump with a 147 on the way to the 2024 Wuhan Open final, responded with two frames to create tension at 4-3.
Trump regained control in the closing stages, taking a scrappy eighth frame before sealing victory with a 50.
He will meet either Ding Junhui or Scott Donaldson in the last eight.
"It was very edgy," Trump told BBC Sport.
"I don't quite have that belief at the moment.
"At the start, I was confident. At 4-1, there were a couple of bad shots, and after that, because I have not been winning, I just became a little bit hesitant and looking for trouble a little bit.
"I feel good with that cue. The table was perfect. If you are a tiny bit off they don't go in."
Selby a contender
On the other table, Mark Selby looked every bit a contender for a third UK title, beating Zhou Yuelong 6-2 to reach the quarter-finals.
Selby had already dispatched Scottish Open champion Lei Peifan and absorbed Zhou's early momentum following the qualifier's surprise win over Ronnie O'Sullivan in round one.
At 2-2 the match was level, although a lengthy 53-minute frame swung Selby's way and set up breaks of 71, 75 and 56 that carried him through.
He awaits either Barry Hawkins or Elliot Slessor.
"If you can't get confidence from beating Ronnie O'Sullivan in a big match you never will do," Selby said afterwards.
"It is getting harder to win Triple Crown titles. It was hard anyway, but it is even harder now with the standard."