Stefanos Tsitsipas sailed into the semis at his first ATP Tour Finals, beating Alexander Zverev 6-2 6-2.
Zverev looked unbeatable on serve against Rafael Nadal on Sunday, but the German was broken in the eighth game after opting to serve and volley on a second serve, making a mess of a low volley and from that point on it was one-way traffic.
Tsitsipas was unrelenting as he broke early in the second before securing a double break to storm into a 5-2 lead over his German rival, before another excellent service game to love - topped off with an ace - sealed the set and match at the O2 Arena.
21-year-old Tsitsipas - the first Greek player to qualify for the elite eight-man season ender - now joins Dominic Thiem in the semi-finals.
Nadal and Zverev will fight for a spot in the last four on Friday, while Novak Djokovic and Roger Federer go head-to-head tomorrow, each needing a win to progress.