Last year's runner-up and seventh seed Ruud will now advance to the semi-finals, where he could face fourth seed Alexander Zverev or 11th seed Alex de Minaur.
Novak Djokovic's disappointing season continued on Tuesday when the defending French Open champion was forced to pull out of the Grand Slam before his quarter-final due to a knee issue.
The injury, sustained in his fourth-round win over Francisco Cerundolo, ended the Serb's quest for a record-extending 25th Grand Slam trophy and will result in him losing the world number one ranking to Jannik Sinner later this month.
"Due to a torn medial meniscus in his right knee (discovered during an MRI scan), Djokovic, who was supposed to play Casper Ruud in the quarter-finals tomorrow, has been forced to withdraw from the Roland Garros tournament," organisers said.
Last year's runner-up and seventh seed Ruud will now advance to the semi-finals, where he could face fourth seed Alexander Zverev or 11th seed Alex de Minaur.
Monday's marathon match was the second straight clash in which Djokovic was taken to five sets after his epic against Lorenzo Musetti and he has spent over nine hours on court in the last two rounds at the year's second major.
The Serb said he had been carrying an injury for a couple of weeks and that the problem had flared up when he slipped during his match against Cerundolo.
The injury could also cast doubts over his fitness for next month's Wimbledon.
It has been a season to forget for Djokovic after his bid for a record-extending 11th Australian Open title was ended by eventual champion Sinner in the semi-finals, before he lost to lucky loser Luca Nardi early at Indian Wells.
Having been stunned by Ruud in the Monte Carlo semi-finals, Djokovic was thrashed 6-2, 6-3 by Alejandro Tabilo in the third round in Rome last month, two days after being hit on the head by a fan's water bottle while signing autographs.
Djokovic is still without a trophy this season after one of the most dominant seasons of his career in 2023 during which he claimed three of the four Grand Slam titles.
