Kevin Anderson had no answer to Novak Djokovic as the world number one marched into the championship match at the ATP Finals.
Roger Federer said Alexander Zverev had no need to apologise for an incident which did not go down well with the crowd in London.
A tie-break controversy marred the end of Alexander Zverev's ATP Finals win against Roger Federer, but the German has apologised.
Roger Federer was knocked out of the ATP Finals in the last four for the second year in a row, with Alexander Zverev progressing.
Novak Djokovic's place in the last four of the ATP Finals had already been confirmed and he defeated Marin Cilic 7-6 (9-7) 6-2 on Friday.
After failing to get out of his group in 2017, Alexander Zverev made sure he qualified for the last four of the ATP Finals this year.
John Isner put up a fight but was unable to prevent Alexander Zverev joining Novak Djokovic in qualifying from Group Guga Kuerten.
Despite losing his first match at the ATP Finals, Roger Federer advanced to the last four thanks to a straight-sets win over Kevin Anderson.
A 6-4 6-3 victory over Kevin Anderson ensured Roger Federer progressed to the last four of the ATP Finals for the 15th time in his career.
Teams from 24 countries will feature in the ATP Cup, which will be held at the start of the season across 10 days in Australia.