Carlos Alcaraz overcame his illness by beating Andrey Rublev at the ATP Finals in Turin.
The 21-year-old Spaniard won 6-3 7-6 (10-8), despite having to interrupt his training on Tuesday after suffering from a chest complaint.
German second seed Alexander Zverev later claimed his second win with a 7-6 (7-3) 6-3 victory over Casper Ruud in the John Newcombe Group.
Zverev guarantees his place in the semi-finals if he beats Alcaraz in the final round of the group stage on Friday.
Wearing a pink nose strip to aid his breathing, third seed Alcaraz won four consecutive matches against Rublev, taking the first set in 37 minutes with a double break of service.
He claimed second place in an epic tie-break, hitting a total of 33 winners in the match, compared to 14 for the Russian.
“I surprised myself with the way I played from the baseline. I was very calm,” Alcaraz said.
“I just focused on my game and what I have to do and forgot that I wasn’t feeling well.”
Alcaraz scrawled ‘Valencia’ on a TV monitor after his victory to show support for fellow countrymen affected by recent floods.
The French Open and Wimbledon champion was out of trouble when he lost his first match in Turin to Norwegian Casper Ruud on Monday.
And although he rubbed his chest during breaks in play on Wednesday, his movement was much sharper.
He came from 40-15 down to break a 4-3 lead in the opening set after Rublev whistled a simple forehand on deuce.
And the Russian squandered a 30-0 lead in his next service game as his opponent secured the set.
Rublev, who lost his opening match to German Alexander Zverev as the eighth seed, held serve to force a tie-break in the second.
An early break gave Alcaraz the lead, although he was put back and had to save two set points before taking the win.
Rublev has now lost six straight ATP Finals matches in straight sets since reaching the semi-finals in 2022.
Two-time champion Zverev wins again
Zverev, seeking his third ATP Finals title, followed up his defeat of Rublev with an outright victory against Norwegian sixth seed Ruud.
Ruud missed just six of his first serves in the first set, but Zverev dominated the tiebreak after the first twelve games produced no break points.
With an hour and twenty minutes to go, Zverev finally made a break to lead 5–3 in the second set and served for the love of victory.
“I think it was actually a really good game from both of us,” he said. “I’m happy I won today, but it was very close.”
On Thursday, American Taylor Fritz will take on Australian Alex de Minuar (13:00 GMT) in the final round of the Ilie Nastase Group matches.
World number one Jannik Sinner will reach the last four if he claims a set against Daniil Medvedev (7.30pm), and will top the group with a third straight win.
