Almost 12 hours altogether so far.
Thiem, who has played three straight four-setters, beat No 19 Kei Nishikori of Japan 6-2, 6-0, 5-7, 6-4 on Court Philippe Chatrier. Nishikori won just 14 points in the first set and nine in the second, losing to Thiem for the first time.
The 24-year-old Thiem leads 4-2 overall against Zverev, but lost to him last month in the Madrid Masters final. Thiem called their upcoming quarter-final "the match-up most of the fans in Germany and Austria were hoping for when they saw the [French Open] draw".
On Court Suzanne Lenglen, Zverev's smash at the net gave him match point, which he took when Khachanov netted a forehand.
Zverev sank to his knees in celebration. He then patted Khachanov, a friend from their junior days, sympathetically on the chest before pumping his arms in celebration.
Zverev has youth on his side but he has been spending a long time on court and his body is showing it. At the start of the fifth set, he had a medical timeout for treatment for an apparent blister on his left foot. It did not seem to impede him, however. He broke immediately for a 1-0 lead and jumped around on court, waving his arms to get the crowd going.
In women's play, 13th-seeded Madison Keys reached the last eight for the first time at Roland Garros after beating No 31 Mihaela Buzarnescu of Romania 6-1, 6-4.
"I made the final of the US Open but lost to my friend Sloane Stephens," Keys said. "Hopefully big things can happen here."
Keys next faces unseeded Yulia Putintseva of Kazakhstan, who beat 26th-seeded Czech Barbora Strycova 6-4, 6-3. If she wins that match, Keys could meet Stephens in the semifinals.
Stephens, the American seeded 10th, also reached the last eight in Paris for the first time, beating No 25 Anett Kontaveit of Estonia 6-2, 6-0.
Stephens next plays Australian Open champion Caroline Wozniacki - seeded second - or 14th-seeded Russian Daria Kasatkina, who finished their match overnight.
- AP