Van Aert (Jumbo-Visma) attacked with five kilometres of the 170km route left but was chased down with the finish line in sight by Uno-X Pro Cycling's Rasmus Tiller (27) who took the win.
In the end, Danny van Poppel (30, Bora Hansgrohe) finished second with Britain's Stephen Williams (27) third and the chasing pack - which included Van Aert - all given the same time.
"Today I overestimated the course a bit," Van Aert said. "We were still far from (catching) the breakaway with 40 kilometres remaining when we finally got some help from Movistar. They were going quite fast into the steep, final climb.
"I thought attack was the best form of defence, so that's what I did. Unfortunately, I isolated myself with guys on my wheel who wouldn't cooperate - that was a bit of a mistake.
"I felt my legs running out of gas, which is normal after seven days of pulling on the front."
Van Aert has a slender three-second lead going into the final stage from Margam Country Park to Caerphilly in Wales, ahead of 10 riders including Van Poppel and Tiller.
"I'm still proud of (my teammates) and hopefully they can hold on for one more day so I can pay them back with a nice GC victory," Van Aert said.
