Thiem knocked out American Jack Sock in three sets, while Khachanov beat a suboptimal Alex Zverev.
PARIS — Dominic Thiem beat defending champion Jack Sock 4-6, 6-4, 6-4 to reach the Paris Masters semifinals on Friday.
The Austrian served out the match and clinched victory on his first match point when Sock hit a big forehand long.
Thiem broke the American in the fifth game of the third set, and then showed good composure to save two break points of his own at 15-40 in the eighth game.
Four-time champion Novak Djokovic and Roger Federer, who is seeking a 100th career title, were playing their quarterfinals later Friday.
Djokovic, who is guaranteed to take the No. 1 ranking on Monday, played Marin Cilic, and Federer faced Kei Nishikori.
The sixth-seeded Thiem, meanwhile, next faces unseeded Karen Khachanov of Russia.
Khachanov, who won the Kremlin Cup in Moscow last month, broke fourth-seeded Alexander Zverev's serve six times in a 6-1, 6-2 win.
Zverev had treatment on his right shoulder late in the first set and made a string of unforced errors. He took exception to some comments made in Khachanov's box and spoke to his opponent at the end of the first set.
''I told him my shoulder is hurting and that his uncle, or something, that he's going a bit crazy. I told him to tell him to keep it down a little bit because he was going a little bit too much over the edge,'' Zverev said. ''I was never going to win the match, so I told him to calm down a little bit.''
Zverev said he was struggling all week with his troublesome right shoulder.
''I did hours of treatment. And after yesterday's match, you know, I had a lot of pain in the evening and during the night. Today, during the warmup, especially on my second serve where I was doing the kick motion, I had a lot of pain.''
Khachanov played down the incident with Zverev.
''When you are playing and you are losing sometimes, you just get a little bit frustrated and upset. And maybe some calls or some noise makes more problems and more frustration for you,'' Khachanov said. ''Everything is fine between us and there is no problem. Just during the match sometimes it's tough to control the emotions.''