The world number one, who has maintained his top ranking since winning the title last year, is trying to avoid going a full year without a trophy for the first time since 2013.
Trump, now reunited with the cue that helped him win the 2019 World Championship, showed flashes of sharpness.
He made a 117 break in the third frame and claimed the fourth frame with a composed 65-point clearance after being behind.
A lengthy 37-minute fifth frame extended his lead to 4-1, but Si, who had previously beaten Trump with a 147 break en route to the 2024 Wuhan Open final, fought back with two frames to narrow the gap to 4-3.
Trump took back control in the final stages, winning a scrappy eighth frame and then secured the match with a 50 break.
He will face either Ding Junhui or Scott Donaldson in the quarter-finals.
"It was quite tense," Trump said to BBC Sport.
"I don’t really have that confidence right now.
"At the start, I felt confident. Leading 4-1, I made a couple of poor shots, and since I haven’t been winning much, I started to hesitate and overthink things.
"I feel comfortable with this cue. The table was perfect. If you are slightly off, the balls just don’t go in."
Selby a strong contender
On the other table, Mark Selby proved he is a strong candidate for a third UK title, defeating Zhou Yuelong 6-2 to reach the quarter-finals.
Selby had already beaten Scottish Open champion Lei Peifan and weathered Zhou’s early challenge after the qualifier’s surprise victory over Ronnie O'Sullivan in the first round.
With the match tied at 2-2, a prolonged 53-minute frame went Selby’s way, followed by breaks of 71, 75, and 56 that secured his progress.
He now awaits either Barry Hawkins or Elliot Slessor.
"If you can’t gain confidence from beating Ronnie O'Sullivan in a major match, you never will," Selby commented afterward.
"Winning Triple Crown titles is becoming more difficult. It was tough before, but the level of competition is even higher now."
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