Alexander Zverev opened his campaign at the ATP Finals in Turin on Monday evening by dispatching Andrey Rublev 6-4, 6-4 in their round-robin opener.
In the afternoon match, Casper Ruud romped through the opening set before fighting back from 5-2 behind in the second to beat an out-sorts Carlos Alcaraz 6-1, 7-5.
Germany's Zverev had last faced Russian Rublev at the same stage of the finals in 2023 and won by the same score.
Zverev, a two-time winner of this event in 2018 and 2021, moved to second in the John Newcombe group, behind Norway's Ruud.
The 27-year-old Zverev, the recent winner of the Paris Masters 1000, has climbed back to No 2 in the world this season. After an injury-hit 2022 season he had briefly dropped out of the top 25 in 2023.
"I was never sure I was going to get back to this level," Zverev said on the court in Turin. "Second year in a row for me here. I still want to get better. I still want to improve on a few things. We'll see how next year goes."
Rublev held to love in his first three service games. He won a 13th straight point on serve to open the crucial seventh game, but Zverev then won the next four to break to 15.
Zverev did not allow a break point against his serve all match and converted immediately both times he had the chance.
"I thought it was a very solid match from my end," said Zverev.
"Andrey is an incredible player, but everyone who plays here is an unbelievable player.
"Against anyone here, you have to play your best to have a chance, you have to be solid, mentally strong. I feel like I did that today, I felt like I used my chances quite well and I'm obviously happy with this win."
AILING ALCARAZ BEATEN
Earlier Alcaraz, the world No 3 got off to a shaky start to his maiden ATP Finals, making 34 unforced errors in an uncharacteristically errant display.
"I'm tired. I'm tired mentally," said Alcaraz, who won the French Open and Wimbledon titles earlier this year.
"Obviously a lot of matches, really tight schedule, really demanding year with not too many days off, not too many periods for you could rest.
"Some players deal with it better than others."
Alcaraz said he had struggled with sickness and adapting to the indoor surface.
"A few days before coming here, I got sick at home," said the Spaniard.
"I don't like to sound like I'm making excuses but today I didn't feel well on court.
"It doesn't matter the times I beat Casper. I have no experience playing on indoor courts. I have to be better on this part of the year that we play on indoor courts."
It was Ruud's first victory over 21-year-old Alcaraz in their five career meetings.
"I knew he was dealing with a bit of a cold. I saw him sniffling in the back area and with a tissue for his nose, so that's a sign that maybe physically he won't be necessarily at 100 per cent. Of course, that is sad, and not good for him, but at the same time it is part of the game."
The Norwegian had suffered seven defeats in his eight matches coming into this tournament.
"This part of the season has not been good for me but hopefully I've saved some wins for this week and I've started well," said Ruud, the 2022 finals runner-up