Six-time champion Roger Federer was “happy and relieved” to see off the challenge of 20-year-old Alexander Zverev and reach the last four at the season-ending ATP Finals in London.
Federer, 36, proved the stronger in the final set, winning 7-6 (8-6) 5-7 6-1.
Germany’s Zverev, seeded third, can still qualify for the semi-finals on his debut appearance at the O2 Arena.
He will play American Jack Sock in his final group match on Thursday, with the winner to progress to the last four.
“That was a tough one for sure,” said Sock.
“It’s been an interesting morning so far, the fire alarm went off at 4am and we had to exit the building. But I love playing here in London, it’s an amazing atmosphere, you make me feel like [I’m] home.
“I’m just excited to win and keep myself alive.”
Federer made it through to the semi-finals for a remarkable 14th time at the season finale, but was pushed hard for two sets by the leading light of the new generation.
Zverev showed his class with some brilliant serving in the first set, making 76% of his first serves, but it was still not enough.
The German had three chances to break in the opening game and would later lead 4-0 in the tie-break, before a rash of forehand errors allowed Federer to come back and take it.
Federer can now look forward to a final group game against Cilic knowing there is only pride, and the wish to maintain his winning run, on the line ahead of Saturday’s semi-finals.
“I’m excited for Alex’s future. He’s a wonderful guy and a great, great player,” said Federer.
“I’m happy and relieved that I can play freely against Cilic, instead of it being a nail-biter. It’s been a tough group so to be through in two matches is great.”