The 2015 champion, Stan Wawrinka, fell to French wildcard Hugo Gaston, who wrapped up the 2-6, 6-3, 6-3, 4-6, 6-0 win with spectators' cries of "Hugo! Hugo!" ringing in his ears. Ranked 239th, Gaston next plays Thiem.
In the women's draw, top-seeded Simona Halep beat Amanda Anisimova 6-0, 6-1. The 19-year-old Anisimova had beaten the 2018 champion in straight sets in the quarter-finals last year.
Thiem, bidding to become the first Austrian with multiple Grand Slam titles, has said he loves the chilly autumnal conditions that other players have grumbled about at this pushed back French Open.
He was broken only once, when he sent a forehand wide to give Ruud a 3-1 lead in the first set. He immediately broke back and converted six of his 15 break points. He also saved seven of the eight break points faced.
Ruud was bidding to become only the second Norwegian to reach the fourth round of a Grand Slam.
The first was his father and coach, Christian Ruud, in 1997 at the Australian Open. He watched on Chatrier. It was again largely empty for that match, victim of the daily limit of 1000 spectators imposed on the tournament because of Covid-19.
The arena built for 15,000 rang out later in the day to shouts of "bravo" and "merci" and saw fans in face masks jumping up and down in waves as Caroline Garcia, of France, beat 16th-seeded Elise Mertens 1-6, 6-4, 7-5. The 45th-ranked Garcia advanced to next face third-seeded Elina Svitolina, who beat the last of 12 Russians in the women's draw, Ekaterina Alexandrova, 6-4, 7-5.
- AP