His struggles continue, but Andy Roddick has survived four rounds of a professional tennis tournament for the first time this year.

Roddick’s 6-3,3-6,6-2 win over qualifier Simon Greul at the Nasdaq-100 Open in Key Biscayne, Fla., put him in the quarterfinals, where he will face David Ferrer, a Spaniard with excellent groundstrokes. If he wins that one, he would likely meet world no. 1 Roger Federer in the semifinals in what would be their second match since last year’s Wimbledon final.

Since losing to surprise finalist Marcos Baghdatis in the fourth round of the Australian Open, Roddick has withstood several more unexpected blows. In San Jose, he won three matches before losing to Scotland’s Andy Murray in the semifinals. In Memphis, he lost his quarterfinals match to Julien Benneteau, a player whom Roddick destroyed in two previous matches, including at this year’s Australian Open. Most recently, in Indian Wells, Calif., he dropped his fourth round match to Igor Andreev.

While none of those opponents is among the world’s most compelling players, Greul, a 24-year-old German, is something less: a man who had a 1-5 record on the pro tour before he qualified for this event. To his credit, he scored several quality victories this week, defeating Paradorn Srichaphan and Tim Henman, both in three sets. Srill, Greul is not about to emerge from obscurity; he is an awkward player with a two-handed backhand that at times can prove deadly. He also grunts quite loudly – as Roddick put it, “It’s amusing when you’re winning and annoying when you’re losing.”

As much as Greul, even playing his best, should not have given Roddick much of a challenge, it was encouraging to see Roddick remain calm as the third set progressed. He even brushed off two instant replays, requested by Greul, that went in his opponent’s favor (more on that below). Afterward, he talked about suppressing negative thoughts this week and about playing more relaxed tennis – two more positives.

Now for Roddick to improve his play, which looks about the same as it has all season: timid. It seems he will never abandon his strategy of camping out 10 feet (or more) behind the baseline, both on second serves and during most rallies. Too often, Roddick plays like the professional equivalent of the pusher who frustrates local club players with looping strokes and lots of spin, except that pushing does not work in the pros, unless one moves like Rafael Nadal (Roddick does not, and for the record, Nadal does not push).

Where is the aggression Roddick spoke so much about before the Australian Open? So far this week, he’s hit 59 winners (including service winners) compared to 50 for his inferior opponents. When Roddick does win, he continues to do it with exceptional serving and a large helping of errors from his opponents, who have made 93 errors this week to Roddick’s 45.

Yes, winning a match on an opponent’s errors is a fine strategy, but not for Roddick. He does not move well enough and stands too far from the baseline – hence giving up too many sharp angles – to defeat the game’s best players with patience. He needs to dictate play and make his opponents feel uncomfortable.

Instead, he remains predictable – crosscourt backhands, heavy topspin forehands, and a slice backhand that he telegraphs with an altered backswing. Thankfully, a long season remains ahead, including Roddick’s best Slam (Wimbledon) and his beloved American hard courts. Perhaps his brother John, now acting as Roddick’s coach, and a few more tough losses will bring out the best in Roddick before the year is out.

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ANOTHER SUCCESS FOR INSTANT REPLAY Simon Greul did not have enough game to upset Roddick yesterday, but few will question that he has two of the very best eyes on tour.

Greul successfully challenged two consecutive calls against Roddick in the third set of his 6-3, 3-6, 6-2 defeat. The first occurred with Greul serving at his advantage, with the score 1-4. He fired a serve to the outside of the service box nearest the chair umpire, forcing a wide return from Roddick. No call was made, but the chair umpire, who had a perfect view, immediately overruled, calling the serve out. Greul challenged, and a replay proved him correct. Game to Greul, who raised his arms to celebrate the modest victory.

On the first point of the next game, Roddick fired an ace down the middle. Greul challenged, and a replay showed the serve to be out by at least two inches. Roddick was forced to hit a second serve, and Greul won the point.

While these replays had no effect on the outcome of the match, tennis’s first live experiment with this technology continues to show how valuable it will be to the sport. So far, umpires have been wrong on challenges between 25% and 30% of the time, and chair umpires have twice overruled calls and been proven incorrect.

This is hardly an error rate to be proud of, though it is not unexpected, considering the speed of today’s game. Frankly, one could not ask for much better from the human eye. After yesterday’s match, Roddick credited the job that umpires do, but said he was happy to have replays.

“They have to be – what’s the word I’m looking for? – accountable for something,” he said. “Because [before] they could make a decision and not really care about it. At least they look stupid in front of 15,000 people now if they make the wrong call.”

In reality, replay is not about making people look stupid. It’s about making them smarter – and maybe less willing to overrule a call from atop a chair that, no matter how close to the ball, is often a lot farther away from the truth than it may seem.

tperrotta@nysun.com