Roger Federer crashed out of the 2018 ATP World Tour Finals in straight sets to German Alexander Zverev in Saturday's first semi-final.

The 21-year-old won 7-5, 7-6 (5) to make his first final in the season-ending event.

Zverev claimed the opener with the first break point of the match at the O2 Arena in London, and his clinical serving in the second-set tiebreak proved too much for the 37-year-old Swiss, who remains one short of 100 career titles.

Zverev will take on either Novak Djokovic or Kevin Anderson in the final; the pair play their last-four contest later on Saturday.

The holds of serve rattled past in quick succession in the first set, with Zverev particularly impressing with his speed:

A tiebreak seemed inevitable, but at 6-5 the 21-year-old pounced at the perfect moment. He won three points in a row on Federer's serve—the second via a brilliant forehand down the line—to put the set within his grasp, and he took it at the first attempt after a fine return of serve.

Federer earned the early advantage in the second to lead 2-1 with a glorious backhand having squandered his first two break points. However, Zverev came straight back, pouncing on a mistake from Federer to break serve and draw level again at 2-2.

The youngster was firmly in the ascendancy for the remainder of the set, regularly holding with ease to nudge ahead before piling pressure on Federer's serve.

Despite the fact that he was merely clinging on for much of the set, Federer was able to force a tiebreak and keep his hopes of taking the match to a decider alive.

A tense tiebreaker included a contentious moment when a ball boy behind Federer dropped a ball during a rally, giving Zverev his first serve back when the point had to be replayed:

It was the veteran superstar who was the first to blink as he netted a volley at 5-4 down to hand Zverev two match points.

Federer saved the first, but a brilliantly played service point from Zverev at 6-5 ahead handed him a deserved victory.