NEWPORT, R.I. -- John Isner felt the ball hit the line, so he conceded the point. It turned into a big moment for Jack Sock.
The seventh-seeded Sock advanced to the semifinals of the Hall of Fame Tennis Championships with a 6-4, 7-6 (4) victory over No. 1 seed Isner on Friday.
Next up for the 21-year-old Sock is third-seeded Lleyton Hewitt, who eliminated American Steve Johnson with a 6-4, 6-4 victory. It's the first career ATP Tour semifinal for Sock, who paired with Canadian Vasek Pospisil to win the doubles title at Wimbledon.
Sock and Hewitt will play on Saturday after induction ceremonies honoring the 2014 class into the International Tennis Hall of Fame, which is highlighted by Lindsay Davenport in the recent player category.
Sock and Isner were tied at 1 in the second-set tiebreaker when Sock hit a serve and paused to look at the chair umpire when it was called out. Isner conceded the point. Sock won the next point and never trailed again, winning the final three points of the match.
Sock had conceded a point down 30-0 in the third game of the set.
The two both spoke about being friends earlier in the week. Did it make it easier to do something like that?
"Maybe a little," Sock said of Isner's decision. "When I did it the first time, you can tell. You can feel when you missed it. He's a very professional guy. He won't say much or show much emotion. For him, I was a little bit surprised, being 1-all in the tiebreaker. It was a serve that I felt, you can kind of feel it. I thought I saw white chalk come up. It was a little surprising, but obviously sportsmanlike."
Isner was certain he did the right thing.
"I thought it hit the line," he said. "I thought I saw the ball hit chalk. The ball is in, the ball is in."
Sock won the match when Isner hit a backhand return long.
In the other quarterfinal, Johnson broke in fourth game to go up 3-1, but Hewitt broke right back.
"It was huge," Hewitt said. "He started serving a little better at the start of the second set. That was important because it just stopped his momentum there."
On match point, Johnson, the No. 6 seed, hit a backhand wide and argued with the chair umpire a bit before shaking hands.
The other semifinal is second-seeded Ivo Karlovic of Croatia against Samuel Groth of Australia. Both advanced on Thursday.
Isner won the 2011 and 2012 Hall of Fame titles and had victories in 14 of his previous 15 matches on Newport's grass courts.