The unseeded 20-year-old not only matched his powerful opponent ace for ace en route to a 6-4, 1-6, 6-3, 6-4 victory, but was also an inspired returner of serve, conjuring up 63 winners - 24 more than Roddick.
The world No 54 was cheered on by a boisterous band of fans from Melbourne's Greek community, clad in blue and white and waving Greek and Cypriot flags. Their excitement was infectious; by the final set the crowd in Rod Laver Arena was in uproar, almost unanimously supporting the effervescent underdog.
It was a stunning result for Baghdatis, who will play Ivan Ljubicic of Croatia, the No 7 seed, in his first Grand Slam quarter-final. He has not yet won a career title, and has competed in only five previous majors.
After his bravura performance the flamboyant young Cypriot could not stop smiling. "I think that was one of my best matches ever," he said. "I was just in my own world and playing great tennis."
For Roddick, who won the 2003 US Open but otherwise has yet to fulfil expectations, the outcome was disconcerting. "There are no unanswered questions in my eyes as far as preparation," he said. "So you're left searching a little bit. That's an uneasy feeling."
It was Baghdatis' composure, as well as his talent, that endeared him to the Melbourne Park crowd. After losing the second set, the 2003 Australian Open junior champion could have crumbled. Instead, he remained calm, concentrated on serving well, and reaped the dividends.
Far more experienced men have faltered when serving for a match, particularly on a grand stage, against one of the world's top players. But the Cypriot "just tried to keep cool and finish it off". Two forehand winners and a Roddick backhand error gave him triple match point, and a judiciously placed cross-court forehand sealed matters.
Baghdatis has had his best results at the Australian Open, where he was defeated by Roger Federer last year in the fourth round.
That, presumably, is a result of the backing for him in Melbourne, where he has nine uncles and 21 cousins, mainly on his Lebanese father's side. It seems not to matter that most of his supporters do not understand tennis. "They think it's football," Baghdatis explained. "So when I speak to them, I try to tell them the rules."
Yesterday their chanting was mostly unintelligible, although at times it sounded strangely like, "I lost my car key, I lost my car keys". Asked to translate, the Cypriot said their favourite chant meant, "Marcos, you are crazy with the white T-shirt".
The match was televised live in the Greek half of the divided island, and yesterday, according to Baghdatis, bleary-eyed Cypriots celebrated by dancing in public fountains. How many fans had watched it back home? "Possibly the whole of Cyprus," he replied.
Roddick, who was knocked out of last year's US Open in the first round, made 31 unforced errors, compared with his opponent's 26. He broke serve only in the second set, while the energetic Baghdatis - who served 16 aces to the American's 15 - broke once in each of the other three.
A semi-finalist here last year, Roddick, who had not dropped a set before yesterday, must be cursing himself. With the men's draw missing the defending champion, Marat Safin, as well as Andre Agassi and Rafael Nadal, the tournament was an opportunity to break his Grand Slam drought.
Baghdatis will meet an on-form Ljubicic, who last night beat Thomas Johansson, the 2002 Australian Open champion, in straight sets. It should be an interesting second week. As Roddick said yesterday: "There's one guy [Federer] who has set himself apart, and the rest of the men's tennis is very deep. Anybody can beat anybody on a given day."
The Cyprus effect Holiday island invests in success
* Philios Christodoulou, chairman of the Cyprus Tennis Federation, summed up how big the sport is on the holiday isle: "We have no history in tennis. A few years ago I would say that less than one per cent of the population knew how to play." Tennis has struggled to gain a foothold in Cyprus because it is seenas an élitist sport. The island has 12 tennis clubs and 5,650 licensed players. It has 53 outdoor courts and one indoor court. Cyprus's tennis federation has poured money into infrastructure projects and gives free coaching at a tennis centre in the capital, Nicosia.
Despite this investment, players have to go abroad if they are serious about becoming professional because the level of competition on the island is not high enough. Baghdatis left Cyprus at the age of 14 for a tennis academy in France.
Yet his departure had no impact on Cyprus's affection for him. "We are so proud. This is a deep honour," said Christodoulou.Reuse content