PARIS — His French Open title defense nearing an end, Novak Djokovic stumbled and tumbled to his knees on the red clay, his racket flying from his right hand as his opponent’s backhand zipped past.

Even Djokovic found it hard to fathom how far he’s fallen, only a year removed from leaving Roland Garros as a player nonpareil, the first man in nearly a half-century to win four consecutive Grand Slam titles.

That he departed this time with a surprisingly lopsided 7-6 (5), 6-3, 6-0 quarterfinal loss to sixth-seeded Dominic Thiem of Austria on Wednesday left everyone, including Djokovic, pondering the answers to difficult questions.

Did he give up in the last set? What has happened to his once-impervious play? Can he summon that again? Does he need a break from the grind of the tour?

“It’s a fact that I’m not playing close to my best, and I know that,” Djokovic said after his first straight-set loss at a major since the 2013 Wimbledon final.

Since completing his career Grand Slam at the French Open 12 months ago, Djokovic has participated in four majors in a row without earning a trophy. He also lost his No. 1 ranking to Andy Murray.

Djokovic was runner-up at the U.S. Open but lost in the third round at Wimbledon, the first round at the Rio Olympics and the second round at the Australian Open.

“The win here last year has brought a lot of different emotions. Obviously, it was a thrill and complete fulfillment, I guess,” Djokovic said during a frank news conference. “I have lived on that wave of excitement, I guess, ’til the U.S. Open or so. And at the U.S. Open, I just was emotionally very flat and found myself in a situation that I hadn’t faced before in (my) professional tennis career.”

The 23-year-old Thiem next faces nine-time French Open champion Rafael Nadal, who advanced when No. 20 Pablo Carreno Busta stopped while trailing 6-2, 2-0 after injuring an abdominal muscle late in the first set.

“I mean, it’s a joke how tough it is to win a Slam,” said Thiem, the only player who beat Nadal in one of his 23 clay-court matches this season. “Now I beat Novak. On Friday, (it’s) Nadal. In the finals, there is another top star.”

In the other semifinal, 2016 runner-up Murray will face 2015 champion Stan Wawrinka in a matchup of three-time major title winners. Murray eliminated No. 8 Kei Nishikori 2-6, 6-1, 7-6 (0), 6-1 on Wednesday, while No. 3 Wawrinka won 6-3, 6-3, 6-1 against No. 7 Marin Cilic.

Simona Halep and Karolina Pliskova reached the women’s semifinals. Halep came all the way back from a set and 5-1 down in the second to defeat Elina Svitolina 3-6, 7-6 (6), 6-0. Pliskova beat Caroline Garcia 7-6 (3), 6-4.

With the wind whipping at more than 15 mph and the temperature in the low 50s, Djokovic was out of sorts in so many ways even before that 20-minute third set in which he won only 8 of 34 points. It’s only the second time Djokovic lost a final set by the score of 6-0 in his 937 career tour-level matches.

“Just pretty bad set,” Djokovic said.

But both men thought the match was decided in the first set, when Djokovic held two set points at 5-4, 15-40 on Thiem’s serve. Thiem erased the first with a forehand volley and the other with a service winner that prompted Djokovic to roll his eyes.

Djokovic’s backhand really let him down in the tiebreaker: All seven points won by Thiem ended with that stroke.

In all, Djokovic made nearly twice as many unforced errors, 35, as winners, 18.