Ruud misses early last four chance as Zverev cruises to second win at ATP Finals
Earlier in the day, Carlos Alcaraz had opened his account at the tournament with an entertaining 6-3, 7-6 win over Andrey Rublev.
World number three Alcaraz comfortably defeated Rublev to lift himself off the bottom of the John Newcombe Group and looked in much better form despite struggling with fatigue and illness in the lead-up to the match.
Rublev meanwhile dropped to last place after losing his sixth straight Finals match, although he put up a battle and had two opportunities to win the second set.
Alcaraz winning in straight sets meant that Ruud would have reached the semi-finals with a match to spare if he had beaten two-time Finals champion Alexander Zverev in the day's late match.
However, Zverev put on a punishing display to prevail in straight sets 7-6, 6-3 and all but ensure a spot in the last four with his second win from as many matches.
Zverev took charge of the match in the tie-break at the end of a first set, which before then had gone entirely with serve, and the pair exchanged blows until the German broke in game eight of the second set and then served to love to win the match.
"I think it was a very good match actually from both of us," said Zverev on court.
"I'm happy to win today but it was very, very close. One or two points decided the first set and I thought the level was very high from both of us."
Leader Zverev can eliminate Alcaraz in his final match of the group stage on Friday when Ruud takes on Rublev.
On Thursday, world number one Jannik Sinner will reach the last four if he claims a single set against Daniil Medvedev, and will top the Ilie Nastase Group with a third straight win.
