Trump built an early lead with breaks of 101 and 57 to go up 2-0. Zhao replied with a break of 100 to reduce the deficit to 2-1, but did not win another frame, as Trump added runs of 51, 54 and 72 to move 4-1 ahead.
Zhao had a chance to close the gap in the sixth frame but missed a pink to the centre pocket on a break of 18, and Trump cleared with 49 to win the match.
The result ends Trump's hopes of an immediate return to the top of the world rankings at this event, but confirms his place in the last four for the first time at this tournament. It is his 76th ranking semi-final.
Zhao replaced Trump as world number one last month, ending a two-year spell at the top of the rankings.
"It was a big game, I have not been that up for a game for a while," Trump said afterwards. "You always have something to prove when someone takes the limelight and you want to prove to everyone you've still got it. I wouldn't say it was a grudge game, but there was a bit more on the line than normal.
"Sometimes I need that kind of rivalry, or someone there just to really get me back on the practice table and get that determination back. When you get older and you've been around so long, you can be a bit flat. These are the kind of games you can really get up for.
"I always enjoy being the underdog and I could feel that inside today. It's always nice to beat the player that's number one. The atmosphere tonight was much better, there was a lot more cheering and support out there."
Trump will play Jak Jones in Saturday's semi-final, which starts at 7pm and is played over a possible 11 frames.
Victory would take him to a 32nd ranking title and a second trophy of the season, following his win at the invitational Shanghai Masters last month.
