Toni Nadal, the uncle and former coach of Rafael Nadal, believes the rain and cold of Roland Garros in the autumn plays right into the hands of world number one Novak Djokovic.
In a telephone interview from Mallorca, Toni told AFP that the heavier balls are also tipping the balance of power closer to the Serb who is chasing a second French Open and 18th Grand Slam.
Nadal, the 12-time champion, can equal Roger Federer's all-time mark of 20 majors with victory in the final in Paris on Sunday.
"Djokovic is very solid. I think he's the one who is most favoured by the new balls. It's more difficult for Rafael," said Toni.
Both Djokovic and Nadal have eased into the last eight in Paris for the 14th time and have yet to drop a set.
Djokovic has slipped just 25 games, Nadal two fewer as they remain on course for another championship clash.
As well as the new heavier balls, the overall playing conditions are radically different to when the tournament is traditionally held in May and June.
It has been raining most days with the temperature hovering between 10 and 15 degrees (50 and 59F).
"I think Rafael has been quite good. He arrived with some uncertainties because this is the year he played the least before the tournament," added Toni of a truncated season due to the coronavirus.
Nadal skipped the US Open and his run to the quarter-finals in Rome last month was his first action since February.
"In the third round against Stefano Travaglia he played well. But Sunday against Sebastian Korda, it was too windy to really judge his game," explained Toni.
"As the conditions change, you have to impose your style of play. To win a point, you have to be prepared to play it longer.
"In previous editions, Rafael was very effective on points under rallies of nine shots.
"This year, the rallies are a bit longer. The topspin is not as effective."
On Tuesday, Nadal will face Italian 19-year-old Jannick Sinner who is the first player to reach the quarter-finals on debut since Nadal in 2005.
"He has everything to become one of the best players of the next generation. He is for sure a candidate for world No.1," said Toni Nadal.
"It will be difficult for Rafael, but he has the qualities to win.
"On Sunday, against Alexander Zverev, Sinner was on top the whole match. Zverev didn't play at a very high level, but Sinner is a very good player."