The winger was ruled out for the rest of the season in February but he's now back in contention after recovering a month to six weeks ahead of schedule.
Speaking to The Offside Line, Glasgow assistant coach Kenny Murray said: "DTH is in full swing, back doing everything, so he's fully training and in contention.He's just had full contact out there. He wasn't ready contact-wise for that last game (the win against Edinburgh). It was touch and go, but we just felt it was too much of a risk."
With a three-week gap between that 1872 Cup match and the Ulster game on Friday 17th, the Warriors hoped to play a bounce match earlier this week against another PRO14 team. That didn't work out but Murray is confident Van der Merwe and others who are short of match action will be ready to face Ulster.
"We had plans to try to get a game. They didn't materialise, so we just get on with it. We try to create the intensity here at training and we've got a squad of 50 guys, so we can get 15 on 15 really easily here and it you've got DTH running against Rory Hughes, for example, you're going to get a good bit of collision and contact in that. So, he's ready to go and he'll be in contention."
Van der Merwe isn't guaranteed a starting spot, but Murray is clear on the qualities he brings. "One is his experience of big games, and there's his ball-carrying ability, his ability to beat players in attack, and also his defensive ability. He's a good talker in the back three, he reads situations really well and he works well with Hoggy (Stuart Hogg), so it will be interesting to see who gets that starting position."
Lee Jones is also available for a place on the wing after injury, and Nikola Matawalu should also be able to play despite sitting out training on Thursday because of a leg knock. In the pack, meanwhile, several of those who have been sidelined should make it, including co-captain Callum Gibbins, who missed the Edinburgh match after failing part of his return-to-play protocol following concussion. "He's passed it," Murray added. "I think it was the day after we played he passed it, so he timed it quite well to get a bit of a rest.
"Fraser Brown came off (against Edinburgh). He had a foot injury. He's again on track to play against Ulster - we're just managing him this week to make sure he's not overdoing it.
"Guys who have not been involved recently - Oli Kebble, Siua Halanukonuka - have been doing some rehab, some training, some running some ball work. So at the moment they're potentially on track to play against Ulster, and we have another week to see how they go.
"The good thing for us now is that we've got more players available going into the semi-final than we've had the last four or five weeks, which is excellent going into a play-off game. It comes down to performance, who's playing well and who's training well.
"We've tried to get real competition in training. It's sometimes a real challenge, because you've got a three-week period to prepare for the game. You don't want to over-train guys and they get a wee bit stale, so we've tried to create a lot of competition at training to make sure there's a real edge and a real buzz and also makes sure everybody knows they're in with a real shot of getting a chance."