Roger Federer held young pretender Alexander Zverev at bay in a gripping contest on Tuesday to reach the last four of the ATP Finals and stay on course for a seventh end-of-season crown.
The world number two saw off a spirited challenge from the German 7-6(6), 5-7, 6-1 in front of a boisterous pro-Federer crowd to become the first man through to the last four.
Both players were scratchy on serve at the start but were quickly into the groove in a tight first set that saw no breaks of serve. The 20-year-old third seed raced into a 4-0 advantage only to see the Swiss charge back to lead 5-4. Zverev earned a set point of his own at 6-5 but could not convert and Federer made his third set point count.
The 19-time Grand Slam champion broke in the first game of the second set to establish an iron grip on the round-robin match, but Zverev hit back in the fourth to get back on level terms.
The Swiss re-found his focus and some consistency on serve in the decider, earning three breaks to close it out 6-1.
On Wednesday, Grigor Dimitrov entered the semifinals with a 6-0, 6-2 demolition of David Goffin.
The results: Pete Sampras group: Grigor Dimitrov bt David Goffin 6-0, 6-2; Boris Becker Group: Roger Federer bt Alexander Zverev 7-6(6), 5-7, 6-1.