LONDON (Reuters) - Alexander Zverev's long-predicted arrival at the summit of men's tennis is taking too long for some impatient observers but he took another significant step forward by beating John Isner to reach the last four at the ATP Finals.
His 7-6(5) 6-3 victory over the American meant the 21-year-old became the first German since Rainer Schuettler in 2003 to reach the semi-finals of the prestigious year-ending tournament and the youngest since Argentine Juan Martin del Potro in 2009.
Zverev's second group victory earned him the runner-up spot behind world number one Novak Djokovic .
"It's obviously great getting to the semi-finals. But the tournament isn't over," Zverev, 21, said. "There's only good opponents left. There's only the best in the world.
The other semi-final will be between five-times champion Djokovic, who beat Zverev comfortably earlier this week, and South African debutant Kevin Anderson.
Djokovic completed group play later on Friday when he beat Marin Cilic 7-6(7) 6-2 in a dead rubber to stay on course for the $2.7 million on offer for an undefeated champion.
Not easing up despite having sealed top spot in the group, Djokovic won 31 consecutive points on serve during the match.
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***** Germany's Alexander Zverev in action during his group stage match against John Isner of the U.S