Roger Federer's former coach Ivan Ljubicic has warned Jannik Sinner could struggle to regain peak condition for his title defence at the US Open next week.
Defending champion Sinner's preparations for the upcoming tournament have been thrown into doubt after illness forced him to withdraw from the Cincinnati Open final against Carlos Alcaraz on Monday.
The world No1 managed just 23 minutes on court before retiring while trailing 5-0 in the opening set.
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The Croatian former coach believes a swift recovery within two to three days is crucial for Sinner's chances, cautioning that extended rest could prove problematic for the four-time Grand Slam champion.
The world No1 was forced to retire just 23 minutes into the Cincinnati Open final earlier in the week
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Speaking to Gazzetta dello Sport, Ljubicic explained prolonged rest could severely hamper the 23-year-old's prospects.
"If he recovers in two or three days, there won't be any setbacks. If, however, he has to rest for the entire week, then it becomes difficult to regain the form needed to win a Slam," he stated.
The former world No3 highlighted additional challenges awaiting Sinner in New York.
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Ivan Ljubicic, who coached Roger Federer between 2016 and 2022, believes Jannik Sinner's recovery is crucial to whether he is able to defend his US Open title
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"Even though Jannik will have more manageable opponents in the early rounds, the distance of three out of five sets, the heat, and the humidity of Flushing Meadows could cause him to lose a lot of energy," Ljubicic noted.
The Cincinnati triumph has strengthened Alcaraz's position in the ATP rankings battle, with the Spaniard holding a substantial lead of nearly 2,000 points in the race for year-end No1.
The 22-year-old claimed his eighth Masters 1000 title in Ohio and could overtake Sinner's current ranking if he secures victory at Flushing Meadows.
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Ljubicic assessed the rankings scenario, telling La Gazzetta dello Sport: "If he wins the US Open and Sinner loses in the early rounds, it will be difficult for Jannik to recover."
The Croatian added that despite potential early losses in New York, he still favours Sinner for the year-end top spot.
The Italian and Spaniard have dominated men's tennis throughout 2024, capturing all seven Grand Slam titles between them.
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One of Jannik Sinner and Carlos Alcaraz have prevailed in the last seven Grand Slams
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Their rivalry has produced consistently tight encounters, with Alcaraz holding a 5-2 advantage in hard-court meetings according to Ljubicic's assessment.
Both players reached the Cincinnati final demonstrating their superiority over the field, before Sinner's retirement.
"They're certainly clearly ahead of everyone else; I'd be surprised if they didn't make it to the final, with Jannik's health still a question mark," Ljubicic observed.
The Croatian anticipates the rankings race will extend beyond New York, noting that autumn events including the Paris Masters and ATP Finals offer 2,500 ranking points.