Novak Djokovic made it two wins from as many matches at the ATP Finals with a 6-4 6-1 victory over Alexander Zverev.
The world number one was pushed harder by 21-year-old Zverev than he had been during his routine win over John Isner on Monday.
Zverev, the third seed, even forced two break points at 4-4 in the first set.
However, Djokovic fended them off before breaking to take the set, and Zverev's challenge promptly evaporated in the second as the Serbian moved a step closer to a sixth title at London's 02 Arena.
"I don't think it was breathtaking tennis from both of us, but a win is a win," said Djokovic.
"Midway through the second set I started to swing through the ball and he made some unforced errors which allowed me to win.
"I didn't want to be 5-4 and a break down playing against Zverev's serve, it was really hard to read his serve, more difficult than Isner. But in the second set it was more easy to read him."
Djokovic's place in the semi-finals was confirmed after Marin Cilic beat Isner in the evening match.
Big-serving Isner took the first set on a tie-break but Cilic came back to secure only his second victory in 11 attempts at the 02, 6-7 (2) 6-3 6-4.
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