Daniel Altmaier will play Frances Tiafoe for a spot in the Halle Open final after the home hopeful defeated Daniil Medvedev in three sets on Friday. 

Altmaier, who booked his place in his first tour-level 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. 

And 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."

And Altmaier's reward is a clash with Tiafoe, who saved five match points in a three-set thriller with second 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."

Altmaier finds his groove in Halle 

After his first-round win over Nikoloz Basilashvili, Altmaier had to come through two three-setters against Hubert Hurkacz and Medvedev to reach the semi-finals at a tournament he had not won a match in his three previous appearances. 

But in doing so, he became only the fourth wildcard, after David Prinosil, Tommy Haas and Nick Kyrgios, to reach the semi-finals in Halle. Should he go all the way, he would be the first German to win the tournament since Haas beat Roger Federer in the 2012 showpiece. 

For Tiafoe, meanwhile, he was put through his paces by Auger-Aliassime, with the Canadian firing 25 aces to Tiafoe's five, while the second seed also won 81% (57/70) of his first-serve points in the match.