WASHINGTON -- This is the time of year that's generally been kindest to 6-foot-9 John Isner and his big serve.
Pounding balls at up to 142 mph, the highest-ranked American man hit 18 aces and never faced a break point Friday during a 6-7 (5), 6-4, 6-4 victory over Marcos Baghdatis of Cyprus that put Isner in the Citi Open semifinals.
Isner won for the 10th time in his last 11 matches as he gets ready for the U.S. Open, which starts Aug. 26 on hard courts similar to those used in Washington.
"I'm in a good rhythm and I'm serving well," said Isner, who has 49 aces and was broken only once in three matches this week. "I'd like to think I can hold my serve no matter what's under my feet, but I do think this (surface) helps."
The 20th-ranked and eighth-seeded Isner, who won the hard-court title at Atlanta last week, will face unseeded Dmitry Tursunov of Russia for a berth in Sunday's final.
Earlier Friday, with the temperature reaching the high 80s, the 61st-ranked Tursunov overcame 13 double-faults with the help of 15 aces to beat a cramping Marinko Matosevic of Australia 6-3, 4-6, 7-6 (4).
Saturday's match against Isner -- who won the longest match in tennis history, 70-68 in the fifth set at Wimbledon in 2010 -- will be Tursunov's second semifinal of the season, and his first at Washington since 2006.
The semifinal on the other side of the men's draw pits top-seeded and two-time champion Juan Martin del Potro of Argentina against No. 3-seeded Tommy Haas of Germany.
Del Potro, winner of the 2009 U.S. Open, grabbed 15 consecutive points during one stretch and defeated No. 7-seeded Kevin Anderson of South Africa 7-6 (0), 6-3 in Friday night's last match. Like Isner, del Potro did not face a break point in the quarterfinals.
The 35-year-old Haas, meanwhile, beat No. 11 Grigor Dimitrov of Bulgaria 7-6 (5), 7-6 (3).
Tursunov now gets the unenviable task of trying deal with Isner's serve, which already is tough enough to handle on any surface, but becomes even more dangerous on Washington's particularly speedy and high-bouncing hard courts. Isner was the runner-up to Andy Roddick here in 2007.
Nine of Isner's 14 career appearances in tournament finals -- and four of his seven titles -- came on hard courts. His three deepest runs at Grand Slam tournaments -- one quarterfinal appearance and two fourth-round showings -- came on hard courts.
"I mean, it's a different sport," 2006 Australian Open finalist Baghdatis said about facing Isner, who has won all five of their head-to-head meetings. "It's a completely different thing. Basically you have no rhythm. You play a rally every, maybe, every five minutes. ... Everything's coming really fast."
Isner won 69 of 82 service points against the 16th-seeded Baghdatis, including the last 17 in a row.
In the first set, though, Isner faltered ever-so-slightly on the final point. That's when Baghdatis managed to return a 138 mph serve deep, and an off-balance Isner pushed a forehand wide. It was a minor blip, but enough to cede the set.
From there, it was Baghdatis who got shaky. At 3-all in the second, Baghdatis went ahead 30-love, then dropped four points in a row, including a double-fault to get broken. That was all Isner needed to pull even at a set apiece, capping it with a 140 mph ace.
The third set followed a similar script: Serving at 3-all, Baghdatis got broken by dropping four points in a row, except this time it was at love after he opened the game with consecutive double-faults.
Serving for the match at 5-4, Isner hit an ace at 132 mph, a service winner at 140 mph, an ace at 142 mph, and then Baghdatis ended it with a backhand into the net.
"You feel, like, under pressure all the time," Baghdatis said. "You feel like you cannot give a point away because you cannot come back if you lose your serve."
Defending women's champion Magdalena Rybarikova of Slovakia returned to the semifinals by beating top-seeded Angelique Kerber of Germany 7-6 (0), 3-6, 6-3, and now will play No. 3 Ekaterina Makarova of Russia. No. 4 Alize Cornet of France meets 64th-ranked Andrea Petkovic of Germany in the other semifinal.
Makarova played only four points -- winning all of them -- and moved on when her quarterfinal opponent, Monica Niculescu of Romania, stopped after one game because of a left wrist injury. Cornet eliminated No. 5 Sorana Cirstea 7-6 (5), 6-2, and Petkovic advanced with a 4-6, 6-1, 6-3 victory over 79th-ranked Paula Ormaechea of Argentina.
Petkovic is a former top-10 player whose career was sidetracked by injuries, including a stress fracture in her lower back and ankle and knee operations.
"I'm just so grateful for the second chance," Petkovic said. "I really think of it as a second chance."