The world number two has yet to drop a set at the tournament and is fully justifying the tag of title favourite he adopted following Novak Djokovic's shock loss to Sam Querrey.
Murray had been expected to face a test from Nick Kyrgios in the fourth round but brushed him aside.
Smith said: "I think he's playing his best tennis of his life. He's continued to improve so many areas of his game.
ATP Antwerp
Murray handed tough opening test in European Open
"Even from when he won here and the US Open, you look at his game and his second serve in particular is so much improved.
"His net play, his confidence going forwards - you just look at the stats from this week and he's come forwards with a lot of success. And the aggression of his game. He's playing closer to the baseline, dictating a lot of the points.
"His clay-court run was amazing so he's playing really, really well. That's the thing with him, and the other top guys as well. They're just striving to get better and better, and he's certainly done that."
Smith on Tuesday named Murray in his team for next week's Davis Cup quarter-final against Serbia in Belgrade.
Murray has consistently said he will play as long as he feels fit and healthy, and Smith is hopeful that will be the case.

Jo-Wilfried Tsonga

Image credit: AFP

He certainly has not been overly taxed physically or mentally so far, maintaining an impressive level of focus throughout his matches.
Coach Ivan Lendl can probably take a lot of credit for that. It was noticeable how much Murray's on-court behaviour improved during their first spell working together, and it appears to be the same again.
Smith said: "Andy would have known what he was looking for in his coaching team and there's no way he'd have approached Ivan if he didn't feel there was something to gain from it, and it looks already (like that's the case).
"(Facing Kyrgios), people talked about the stress of that one, but he was so focused from start to finish. We didn't see him expend any energy on anything else apart from the tennis ball and the points. I thought he looked really in control of everything and there's no doubt that Ivan will have added to this, and that's why he's here.
"Obviously it was the best stage of his career when he was with Ivan last time and let's hope it can repeat itself. He obviously feels Ivan has that aura around him that enables him to stay focused. I'm not privy to the conversations they have in private and I'm sure those are of as much value as when Ivan's watching the match."
Murray has an excellent record against 12th seed Tsonga having beaten him 12 times from 14 meetings.
Two of those matches came at Wimbledon, with Murray defeating the Frenchman in the quarter-finals in 2010 and the semi-finals two years later.
"Both matches were hard," said Murray. "They weren't easy. A few of the sets came down to just a couple of points.
"I know Tsonga is one of the best grass-court players in the world. If he plays well, I'm not on my game, I can lose that match for sure."
Murray set to drop out of ATP top 170
14/10/2021 AT 11:27
ATP Indian Wells
Murray after Zverev loss: I don’t deserve Davis Cup spot
13/10/2021 AT 07:57