Marshall was an 11-2, 8-3 winner over Julie Forrest from Teviotdale, the only woman to have qualified for the main singles event while Foster was pushed to the limit by Daniel Salmon, a left-handed teenager from Cardiff.
Forrest held a double on the first end against Marshall but the Tranent man's running shot converted it to a 3 in his favour. Forrest, who beat six English players to win the qualifying event at Carlisle, competed well throughout and forced Marshall to play some big shots but he came through confidently. She said: "I played well enough in patches but wasn't consistent enough overall."
Marshall said: "She played well, put me under pressure at times but had some bad results at other times. It's the best opening performance I've had in a long time. I normally scratch about in the first round but, with a good run in the pairs under my belt [he and Marshall are through to the final, bidding for a fifth successive title], I feel confident."
Salmon won the first set 7-4 from Foster with a superb final bowl but the Scot took charge and drew level with a 10-3 second-set win and won two tense tie-break ends for a 2-0 victory. Foster said: "Daniel was erratic in the trial ends but, when the game started, he was deadly.
"In the first tie-break [end] I thought he was coming in with his last bowl for shot but it crept a few centimetres too far; in the second I had to play a massive bowl to get the shot. Daniel's last bowl came in to take shot but it fell out of the count; that's how tight the match was."
The mainstream singles has a break for a few days, while the mixed pairs and ladies' matchplay singles take centre stage. Scots are involved in each of today's mixed pairs quarter-finals. Foster, the defending champion, teams up with Laura Thomas, from Wales, and David Gourlay partners Cumbria's Eleanor Gass, in the top half of the draw. Last year's beaten finalists, Darren Burnett and Australian world champion Karen Murphy, are in the bottom half, as are Alex Marshall and Guernsey's Alison Merrien.