INDIAN WELLS, Calif. -- Jack Sock is getting quite the workout at the BNP Paribas Open, stretching all four of his matches to three sets on his way to the semifinals.
The 24-year-old American notched his biggest singles victory yet with a 6-3, 2-6, 6-2 quarterfinal win over Kei Nishikori on Friday. Sock finally broke through against a top-five opponent after losing his first seven matches against such competition.
"The doubters in the past can't really say much anymore," he said. "I actually feel almost stronger as the tournament has gone on."
Sock will need to keep up his strength for his semifinal against four-time Indian Wells champion Roger Federer. He lost to Federer in straight sets in the fourth round of the 2015 tournament.
"When I played him here last and I played him in Basel a couple years ago, I don't think I went on the court giving myself a ton of chances of winning. That's changed, for sure," Sock said. "I am playing confident tennis as well. If I go out there and play the right tennis and play the right strategy, I can give myself a shot."
Federer advanced when Nick Kyrgios withdrew with what the Australian suspected was food poisoning.
Sock improved to 15-2 and has won titles at Auckland and Delray Beach, Florida, this year. He is the top-ranked U.S. men's player.
"The mental side of it is probably the biggest difference so far this year," he said. "The matches I have been winning this year, especially this tournament, I probably wouldn't have been winning year and a half, two years ago."
Sock overcame four match points against 12th-seeded Grigor Dimitrov in the third round and was two points from losing against Malek Jaziri in the third set before he pulled out a 4-6, 7-6 (1), 7-5 fourth-round victory.
Nishikori had not dropped a set in the tournament until losing the first against Sock. Nishikori, ranked fifth, had won seven straight matches against American opponents.
"I was having really tough time with my return, missing a lot, and giving him a lot of short balls," Nishikori said. "He was attacking really well, first and third sets. Little, lazy mistakes for me. Too many unforced errors, especially in the third to give him too many chances to break."
Federer got an unexpected day off after Kyrgios withdrew less than two hours before they were to take the court. The Aussie tweeted that he "had a restless night of being sick."
He said "to play a great champion like Roger, I need to be at my best to have a chance."
Kyrgios' withdrawal spoiled the highly anticipated match with Federer. Neither player had dropped serve in the tournament and their only other meeting was decided in three tiebreakers at Madrid in 2015.
"I was really looking forward to playing the match," Federer told fans waiting for the match on stadium court. "It's a pity."
"I'm playing great tennis and I didn't expect it because I'm still on the comeback," said Federer, who won his record 18th Grand Slam title at the Australian Open in January after being sidelined by various injuries for much of 2016.
Federer, a four-time winner at Indian Wells, also advanced via a walkover in the quarterfinals in 2008.
Top-ranked Serena Williams withdrew before the tournament began with what she said was a knee injury that will also keep her out of the Miami event next week.