WATCH the ATP Tour LIVE in Australia on beIN SPORTS | Subscribe here and take advantage of our 7-day FREE TRIAL

Altmaier, who booked his place in his first semi-final above ATP 250 level, downed last year's beaten finalist Medvedev with a hard-fought 6-4 6-7 (6-8) 6-4 victory.

The German wildcard started strongly by breaking Medvedev's serve in the opening game of the match, and that was all he needed to take the first set.

Medvedev assumed more control in the second but failed to take his opportunities as he missed three break point chances early on before a tie-break was needed to split the pair.

That went in favour of Medvedev, with the players then trading blows throughout the decider in search of an opening, the first of which went in Altmaier's favour.

But when 5-3 up and serving for the match, Medvedev broke to love. Altmaier soon dusted himself down, though, and returned the favour to progress to the final four in Germany.

"It means a lot and, on the surface, I have not had the greatest results," Altmaier said. "I am really happy to go through it.

"A lot of emotions are coming through my head, but I am really happy right now. I will enjoy that moment."

Altmaier's reward is a clash with Tiafoe, who saved five match points in a three-set thriller with No.2 seed Felix Auger-Aliassime to advance to the final four.

In a two-hour-and-32-minute epic, Tiafoe dug deep to earn a hard-fought 3-6 6-3 7-6 (14-12) victory, as he looks to become the first American to win the competition.

"It was an up-and-down match," Tiafoe said. "Both of us knew we were both going to give it one last push, and I am happy I came through.

"He had his chances, I had my chances. Classic grass-court match."