- Bats Right, Throws Right
- Height 6' 4", Weight 170 lb.
Kiefer debuted in the ABL in 2017 with the Traiskirchen Grasshoppers, hitting .275/.326/.313 and allowing nine runs (six earned) in 12 IP; another of the team's rookies, Philipp Eckel, would join him on the national team just two years later.  In 2018, he batted .267/.323/.360 and was 1-3 with a 7.03 ERA. He tied for 7th in the league with eight doubles.  During 2019, he was 1-1 with a save and a 5.68 ERA while his offense was improving (.361/.459/.443). He missed the ABL top ten in average by .007 and was one of the top ten Austrians. 
At age 17, he made Austria's team for the 2019 European Championship. Unlike prior years, the reports did not list birthdates for players, but he was presumably one of the youngest players in that year's Euros. While he was more prolific as a hitter than a pitcher in the ABL, he was used exclusively on the mound and did very well. In his debut, he relieved Fabian Hirnschal in the 7th with a 12-6 deficit against France. He was far better than his predecessors, retiring Leonel Cespedes, walking Mael Zan and then retiring Dylan Gleeson and getting Felix Brown to hit into a force. In the 8th, he went 1-2-3 against former minor leaguers Maxime Lefevre, Andy Paz and Frédéric Hanvi, the latter two by strikeout. Jacques Boucheron opened the 9th with a single but Kiefer whiffed Bastien Dagneau and Gabriel Fromental-Houle then Zan grounded out. His other game, he started against Great Britain and allowed only one run (an Alex Crosby homer) in six innings (4 H, 1 BB, 4 K) to beat Rei Martinez; Benjamin Salzmann relieved with a 5-1 lead and Moritz Hackl saved it. He led Austria's qualifiers in ERA, .29 ahead of Alex Seidl and Christian Tomsich and his 8 K were second, 3 behind Tomsich. He tied Pavao Karin for 6th in the 2019 Euros in ERA (1.00) and was 4th with a .161 opponent average, between Orlando Yntema and Sascha Koch. 
The tall right-hander continued his development in 2020, producing at a .400/.500/.671 clip with 27 runs and 20 RBI in 20 games while also starring on the hill (6-2, 3.08, 91 K in 52 2/3 IP, .196 opponent average). He was among the ABL's top ten in average (5th, between Heorhii Hvrytishvili and Samuel Hackl), OBP (5th), slugging (tied for 5th), runs (tied for 2nd with Moritz Scheicher, Ferdinand Obed and Salzmann, 6 behind Eckel), hits (34, 2nd, 7 behind Obed), RBI (tied Salzmann for 10th), doubles (6, tied for 6th), home runs (5, tied for 2nd), total bases (57, 1st, one ahead of Obed), ERA (9th), opponent average (3rd, behind two import players), IP (4th, between Hvrytishvili and Ryan Rupp), K (1st, 14 ahead of Hvrytishvili), wins (1st, one ahead of Rupp and Thierry Letouze), appearances (10, tied for 2nd, one behind Scheicher) and starts on the hill (7, tied for 7th).