In a matchup of the two tallest players on the ATP Tour, qualifier and 2007 champion Ivo Karlovic saved two match points in a gripping third set tiebreaker to defeat No. 4 seed John Isner 7-6 (2), 6-7 (2), 7-6 (9) yesterday in the quarterfinals of the U.S. Men’s Clay Court Championship. The 6-foot-10 Karlovic pounded out 24 aces and saved 10 of 11 break points in the 3-hour and 16-minute marathon win over his 6-9 American opponent. Karlovic jumped out to a 5-2 lead in the third set tiebreaker, but squandered two match points at 6-4, the first coming on a double fault. Isner wasted his two match point opportunities at 7-6 and 9-8 and could only watch as Karlovic whistled a forehand winner past him on the Croat’s third match point. There were just two breaks of serve in the match — Isner getting the first one at 5-5 in the second set and Karlovic the other in the very next game. Karlovic won 80 percent of his first service points and 70 percent overall. Isner, who dropped to 1-2 against the 32-year-old Croat, bombed 23 aces and won 84 percent of the points behind his first serve. Karlovic, ranked No. 174, missed more than half of last season after undergoing achilles tendon surgery. The former world No. 14 will be making his first semifinal appearance since February last year (Delray Beach) when he takes on American Ryan Sweeting this afternoon. “Now I know what it’s like to play me,” Karlovic said. “It’s really not easy. I was trying to stay confident even if he was acing me left and right. I tried to stay calm and in the end, I don’t know what to say, I was a little luckier. It was a close match.” Karlovic improved to 10-9 on the year.