However, a late try from Sixways bound this summer Errie Claassens saw the home side set up a grandstand finale with fly half Gordon Ross missing a long range penalty to win the game with the last kick of the game.
The result means Warrior remain undefeated in Pool A and have guaranteed top spot and a home semi-final to the delight of boss Hill who saluted his side for holding firm under huge pressure.
"I thought it was a great effort to come to London Welsh and win," he said. "Looking at last year's statistics, in the group stages nobody went unbeaten. Exeter and Bristol lost games so it is not easy. Every team is up for it and London Welsh were massively.
"To be five from five, it is looking good and now we need a big performance next week to make it six from six. That would be no mean feat at this stage of the season."
Hill was delighted with the first half showing at Old Deer Park as Worcester took a 16-10 lead against the wind, but admitted the team needed to show more composure and make the right decisions in the second period.
He said: "I was very pleased with our first half. Against the wind, we had the tactics spot on to lead 16-10 and I could not fault the players at all.
"We were clinical, accurate and the lead was excellent. We had the wind behind us in the second half and I expected us to use that to put a lot of pressure on them and get away from them. But they put a real fight up.
"London Welsh had a huge amount resting on this because they haven't mathematically qualified yet for a semi-final place. They really wanted to beat us to have a chance of winning the group and avoid going down to the Cornish Pirates in the semi-finals. They were always going to play very well.
"I was disappointed at the start of the second half because, as we thought they would do, they tried to run out of their own 22 against the wind and we turned them over," he added.
"We had the ball for two minutes, got the penalty and should have gone for three points. The players know that and instead we went for a scrum and in the end knocked the ball on and London Welsh relieved the pressure. To go 19-10 up, just after half-time, would have put them under massive pressure.
"There is a lot of pressure on the team and in the last quarter of an hour a little bit of composure was required. We tried to run the ball a bit too much around the halfway line when we just needed, with the wind behind us, to put the ball in the corners.
"Credit to London Welsh, I thought they defended superbly well and they really had to go for it in the last quarter of an hour.
"We needed composure and there wasn't as much as I would have liked. I will look closely at the last part of the game to see how we played."