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 of Tuesday's clash, 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, reports BBC.
Eighth seed Sock earlier beat Croat fifth seed Marin Cilic 5-7 6-2 7-6 (7-4) in the second round of matches in the Boris Becker Group.
"That was a tough one for sure," said Sock.
"It's been an interesting morning so far, the fire alarm went off at 4:00am 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."
In the Pete Sampras Group on Wednesday, Grigor Dimitrov will play David Goffin at 14:00 GMT followed by Dominic Thiem against Pablo Carreno Busta, who replaces the injured Rafael Nadal, at 20:00.
Jamie Murray and Brazil's Bruno Soares take on Marcel Granollers and Ivan Dodig in their second doubles group match at 18:00.