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.
Rafael Nadal is to undergo ankle surgery after revealing he is out of the ATP Finals in London because of injury.
The Spaniard pulled out of the Paris Masters with an abdominal injury and after failing to recover in time, has now withdrawn from next week's event.
He was scheduled to have a procedure to remove a floating body from his ankle on Monday in a bid to be fit for the start of the new season.
In a post on his official Facebook page, Nadal said: "It has been a complicated year, very good at the tennis level when I was able to play, and at the same time very bad as far as injuries are concerned.
"I have done everything possible to reach the end of the season in good condition, both in Paris and London, doing things well and I really wanted to play.
"Unfortunately, I had the abdominal problem in Paris last week and, in addition, I have a free body in the ankle joint that has to be removed in the operating room today.