Carlos Alcaraz overcame illness to beat Andrey Rublev in impressive fashion and revive his ATP Finals hopes with a 6-3 7-6 (10-8) in Turin.
The four-time Grand Slam champion has been suffering with respiratory issues during the week which contributed to an opening round-robin defeat to Casper Ruud on Monday.
But the third seed, bidding to win the season-ending Finals for the first time, looked far sharper against Rublev, condemning his opponent to a second defeat in as many matches as he triumphed in straight sets.
Later on Wednesday, Zverev took a huge step towards a semi-final place with a straight sets win over Ruud in an entertaining contest.
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In the day's first game having sharing the opening six games, Alcaraz crucially broke Rublev's serve with a scorching forehand winner and proceeded to reel off five successive games to seize control of the match.
Rublev, who also lost his opening match to Zverev, managed to stay with Alcaraz in the second set and could have earned a couple of break points at 2-2 had he not missed a routine ball after chasing a drop shot.
Both players served well to take the match into a high-octane tie-break in which Alcaraz had to fend off two set points before sealing victory with his second match point.
Rublev has now lost six successive matches in straight sets at the ATP Finals since reaching the semi-finals in 2022.
Alcaraz is still likely to need to beat world No 2 Zverev in his last group match on Friday in order to qualify for the final four.
Alcaraz: I surprised myself today
Carlos Alcaraz, after his straight-sets win:
"I surprised myself a lot.
"The way I played today from the baseline, I was really calm and tried to just focus on my game and what I had to do - forget that I'm not really feeling well.
"Once you step on the court, you have to forget everything you're struggling with outside the court."
On his final round-robin opponent, Alcaraz said: "Zverev is playing great tennis.
"He is one of the toughest players in the world right now, that's for sure. I will try to play good tennis and enjoy the same way I did today. Then let's see."
Robson: A complete turnaround from Alcaraz
Sky Sports' Laura Robson:
"Alcaraz had a totally different attitude to what we saw the other day
"He didn't necessarily look any better physically, and he doesn't sound healthy, that's for sure. But he dealt with it as best he could and made the most of it.
"He played with enjoyment again, got the crowd involved, went for the fun shots - and that's what was missing against Ruud. A complete turnaround.
"Rublev had his chances, for sure, but couldn't close out that second set."
Tale of the Tape
How the John Newcombe group looks
Ruud defeated by dominant Zverev
World No 3 Alcaraz will take on Zverev on Thursday knowing realistically only a victory will send him through to the semi-final stage.
Zverev is all-but guaranteed a place in the last four after he beat sixth seed Ruud 7-6 (3) 6-3.
A tight first set went the way of Zverev, who sent down a 144mph ace to clinch the opener on a tie-break.
With momentum on his side, new world No 2 Zverev managed to break in the eighth game to follow up his victory over Rublev and sit top of John Newcombe group.
"I do feel quite good on the serve. I do feel like I'm hitting my spots well and my speed is up from the last few weeks as well," said Zverev on Sky Sports Tennis.
"I do feel well in that regard. I thought today's match was better from the baseline than the fist match.
"Casper has been serving incredibly well.
"We've [Zverev and Alcaraz] played some very tough battles this year at Grand Slam events in Australia and at the French Open.
"I'm expecting a good match and for him to play his best tennis yet.
"We'll see how it goes. I'm looking forward to it. We're both in great shape so it should be a good match."
Henman: You can't afford to blink
Sky Sports' Tim Henman:
"I'm trying to think if Casper Ruud has ever played better tennis and lost.
"These types of conditions are out of his comfort zone and he doesn't have so much time to unwind but he played some impressive tennis.
"When it's that level, you can't afford to blink and it proved costly.
"Zverev's serving was just incredible. From Ruud's point of view he'll be disappointed but I think there are lot of positives he can take from this match into a critical match on Friday."
Tale of the Tape
Thursday at the ATP Finals on Sky Sports Tennis
- America's Taylor Fritz takes on Australia's Alex de Minaur (1pm)
- World No 1 Jannik Sinner meets fourth seed Daniil Medvedev (not before 7.30pm)
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