Pain-free Zverev blasts into Australian Open semi-finals
Zverev sent down 24 aces in a 6-3, 6-7 (5/7), 6-1, 7-6 (7/3) victory and faces world number one Carlos Alcaraz or home hope Alex de Minaur for a place in the Melbourne final.
World number three Zverev is desperate to finally win a Grand Slam at age 28, having lost three major finals including being well beaten in last year's title decider by Jannik Sinner.
The German's 2025 season fell away after that, winning just one title in an injury-hit campaign and he also struggled with his mental health, taking a one-month break after losing in the first round at Wimbledon.
"The last 10 days I felt healthy, which is very nice, and pain-free, which I haven't felt in a long time. Probably in 12 months," he said after dismantling Tien.
"I feel like I'm happy on the court because I am playing pain-free and I'm playing a good level."
The quarter-final took place under a closed roof at Rod Laver Arena to fend off temperatures forecast to hit 45C outside.
At 29 in the world, the Californian Tien was the lowest-ranked player left in the men's draw.
He was also the youngest at age 20 and was playing in the biggest match of his life, having never reached the last eight at a major before.
In contrast, Zverev is an experienced campaigner at the deep end of Grand Slams, but has famously never captured one of the four biggest tournaments.
He was also a runner-up at the US Open in 2020 and again at the French Open in 2024.
'Incredible'
The more experienced man made the better start, breaking Tien for a 4-2 lead on the way to clinching the first set.
Tien, who won his first ATP title in November and is coached by the 1989 French Open champion Michael Chang, went toe-to-toe with Zverev in the second set.
With serve dominating they went to a tiebreak, where Zverev upped the ante to go 5-3 up.
But Tien refused to buckle and defended brilliantly, clawing back and then overhauling the deficit to level the match.
The American had the crowd on his side but Zverev's serve was unrelenting and he barrelled into a 5-1 lead in the third set.
Zverev romped through the set in 28 minutes as Tien's unforced error count mounted.
With Chang a vocal presence, Tien regathered in the fourth and had a set point at 6-5, only for Zverev to dig himself out of trouble and force the tiebreak.
With his serve blazing, the German raced through the tiebreak to wrap up victory in just three hours and 10 minutes.
"Without my aces I probably would not have won today," said Zverev, who only had one double fault, on a match point. "Obviously very happy with my serve.
"Learner off the baseline was playing unbelievable," he added. "The way he is playing is incredible."