Trescothick holds up Lancashire
The 40-year-old former England opener was not at his beat and required some good fortune, but remained unbeaten on 63 at lunch, which was taken with Somerset 97 for two, needing a further 83 to avoid an innings defeat.
Kyle Jarvis removed the off stump of nightwatchman Tim Groenewald with the total on 13 and Simon Kerrigan dismissed Tom Abell for ten, taking a return catch off a leading edge to make it 61 for two.
Trescothick was then joined by fellow left-hander Chris Rogers and the pair needed all their experience to combat the spin of Kerrigan, bowling into the rough outside their off stumps.
It was fascinating cricket on a gloriously sunny day and Somerset face a testing afternoon if they are to salvage a draw.