Injured Federer withdraws from Italian Open
The 37-year-old has an injury to his right leg.
Federer, 37, is playing his first clay-court season in three years and paid the price for winning two matches on Thursday as he withdrew from his last-eight match with Stefanos Tsitsipas.
The Swiss, who has never won in the Italian capital, saved two match points against Borna Coric in his second match on Thursday but succumbed to a right leg injury.
He said on the ATP Tour website: “I am disappointed that I will not be able to compete today.
“I am not 100 per cent physically and after consultation with my team, it was determined that I not play.
“Rome has always been one of my favourite cities to visit, and I hope to be back next year.”
Tsitsipas was handed a walkover to the semi-final where he will now meet Nadal.
The Spaniard, the defending champion, cruised past compatriot Fernando Verdasco 6-4 6-0 and will be eyeing a ninth crown in Rome, 14 years after his first.
Del Potro, who fractured his right kneecap at the Shanghai Masters in October, secured his first two wins since February earlier in the week.
The Argentinian world number nine was first to strike in the opening set, breaking Djokovic’s serve in the seventh game and surviving two break points on his own serve in the 10th before holding to take it 6-4 after just over an hour.
Both players had held serve in the second set before Djokovic, leading 3-2, broke Del Potro for the first time in the sixth game and went on to open up a 5-2 lead.
Del Potro was made to dig deep, saving set point in his next service game before holding and then breaking the Serbian’s serve for a second time to level the scores at 5-5 and effectively force the tie-break.
Djokovic came under immense pressure in the tie-break, but refused to yield, saving two successive match points before winning 8-6 to get back on terms at one set-all.
Only one break of serve was needed to settle the tie in the deciding set, with del Potro losing two successive points against serve to go down 3-2 and Djokovic serving out for the match.