BY HOWARD FENDRICH
AP TENNIS WRITER
WASHINGTON -- Even before Ryan Harrison broke his racket by spiking it on the Citi Open court or screamed "Oh, my God!" in exasperation, top-seeded Juan Martin del Potro was feeling pretty good about the way things were going Thursday.
And they kept improving for the 2009 U.S. Open champion.
Back in action after nearly a month off, del Potro won twice in a span of about 7½ hours to reach the Citi Open quarterfinals. Del Potro started by winning a rain-postponed match against 107th-ranked American Harrison 6-1, 7-5, then returned to the court under the lights at night to defeat 14th-seeded Bernard Tomic of Australia 6-3, 6-3. Each lasted about 70 minutes.
"Really good day," del Potro said, smiling as he leaned back in his chair and stretched out his legs. "The match against Tomic was a little better than the first one."
Del Potro, who received a first-round bye, is 11-1 at the hard-court tournament in Washington, where he won titles in 2008 and 2009.
The Argentine, No. 7 in the ATP rankings, hadn't competed since July 5, when he was eliminated in five sets by No. 1 Novak Djokovic in 4 hours, 43 minutes, the longest semifinal in Wimbledon history.
"I was training hard before coming here," said del Potro, who will face No. 7-seeded Kevin Anderson, also a winner twice Thursday.
Anderson eliminated James Duckworth 6-3, 3-6, 6-4, then beat Mardy Fish of the United States 7-6 (2), 6-1. Before that match finished, Fish's withdrawal from next week's tournament in Montreal was announced; the event said he pulled out for personal reasons.
"It's hard for me, right now, to come out and just play every single week," said Fish, who has missed time the past two seasons while dealing with a heart condition.
"I still know I have a long ways to go to get back to where I'd like to be and where I was, maybe, 12 months ago," he added.
The other men's quarterfinals are No. 8 John Isner against No. 16 Marcos Baghdatis, No. 11 Grigor Dimitrov against either No. 3 Tommy Haas or No. 13 Ivan Dodig, and Marinko Matosevic against Dmitry Tursunov.
Matosevic beat No. 4 Milos Raonic of Canada 7-5, 7-6 (7), while Baghdatis surprised No. 2 Kei Nishikori 6-1, 6-2. Like del Potro, Nishikori played twice Thursday.
Del Potro said his left knee felt fine; he wasn't wearing any sort of wrap the way he did at the All England Club after hyperextending it on a fall during a match.
"You never know" how the knee will hold up, said del Potro's coach, Franco Davin. "For now, it's OK."
Harrison was ranked inside the top 50 last season and is widely seen as the next top American male player.
"He has good potential. He serves well. He plays well on hard courts. But sometimes he looks young," said del Potro, who was a couple of weeks shy of his 21st birthday when he won the U.S. Open. "He's still 21. He needs to learn a few things to become a better player in the future."
Harrison trailed 4-0 after only 11 minutes. Both men hit serves at the considerable speed of 130 mph, but the difference was that once the ball was in play, del Potro's groundstrokes were far more consistent.
"Sometimes you go out there and for whatever reason, it's not feeling as well as you want it to," said Harrison, 0-18 against top-10 players. "The best guys, whenever they see someone's off, they just press you, and they just make you feel like you're under duress all the time."
After pushing a forehand long to get broken at 3-0, Harrison smashed his equipment, earning a warning from the chair umpire for racket abuse. During a particularly rough stretch from late in the first set to early in the second, Harrison dropped 13 consecutive points, a drought that ended right after he dumped a backhand in the net, looked to the sky and yelled. From there, he played evenly against del Potro, until getting broken in the last game.
Del Potro thinks Harrison's histrionics help foes.
"It's easier for us, because we ... (see) our opponent really, really low," del Potro said.
Three other American men joined Harrison on the way out: No. 6-seeded Sam Querrey, Jack Sock and Tim Smyczek.
In women's action, the only seeded player to lose was No. 8 Madison Keys of the United States, who failed to convert two match points and bowed out against Monica Niculescu 6-1, 2-6, 7-6 (6). The women's quarterfinals are No. 1-seeded Angelique Kerber vs. No. 7 Magdalena Rybarikova, No. 3 Ekaterina Makarova vs. Niculescu, No. 4 Alize Cornet vs. Sorana Cirstea, and Andrea Petkovic vs. Paula Ormaechea.
The most unusual ending to a match Thursday came when Olga Puchkova of Russia was defaulted by the WTA supervisor after hitting a line judge in the knee with a ball between points. Trailing Ormaechea 3-6, 6-3, 4-1, Puchkova was forced to forfeit even though she said it was an accident.