Hopes that a dazzling second set had restored the 24-year-old from Oxford's momentum to progress to his first quarter-final at Flushing Meadow began to fade after he was broken for 2-3 in the third set amid protests about line calls.
Philippoussis not only won the set, but took a 4-0 lead in the fourth to send Henman reeling. The Briton, who saved four break points at 0-3, salvaged on one more game before double-faulting to go match point down, Philippoussis securing victory after two hours and 10 minutes.
Apart from the second set, during which Henman shot 5-0 ahead after only nine minutes and broke his opponent in thme finalgame after a 30-minute rain delay, Philippoussis gradually raised his game to the level that his made him seem unplayable on certain occasions, even when faced by Pete Sampras.
As Pat Cash, Philippoussis's coach, said after last night's performance: "When Mark plays like that there's no stopping him.
Andre Agassi was denied a duel with Sampras by Karol Kucera, who had never won a match here before this year's championships.
The ninth seed regarded his victory, 6-3, 6-3, 6-7, 1-6, 6-3, as justice after Agassi's antics during the third set the night before. Kucera lost a 4-1 lead as Agassi mocked him by imitating the Slovak's habit of tossing the ball and catching it while attempting to serve in a tricky wind.
Complaining to the umpire, Norm Chryst, and being told that Kucera could not help it, Agassi said: "If I do it every time, would that be all right?" The American deliberately let the ball drop on several serves, rousing the crowd on Court No 2 to support him with greater fervour.
After Agassi forced a tie-break, Kucera recovered from 0-4 and was within two points of winning the match at 5-4. Agassi won the next three points to win the shoot-out 7-5, and led 3-0 in the fourth set when play was suspended overnight because of rain.
Agassi had to save two break points in the fifth game yesterday before completing his comeback from two sets to love down. Kucera, trying not to let the ball fall to the ground even though a tricky wind made this difficult, double-faulted twice to lose the opening game of the fifth set.
Agassi held on for 2-0, his downfall starting after he was unable to break Kucera a second time from 15-40 in the third game. Kucera converted a fifth break point to level at 2-2, and then broke for 4-2. Agassi's last chance went when Kucera saved a break point when serving for the match.
"What Andre did [on Monday night] was a little bit unfair," Kucera said. "My ball toss was not good yesterday, and I had problems with that. And when you are under pressure it comes again. I didn't do it on purpose."
Agassi said: "If he's tossing it and not hitting it, that's distracting. Whether he meant it or not, it's not acceptable."
Pat Rafter, the defending champion, defeated Goran Ivanisevic, the Wimbledon runner-up, 6-3, 6-4, 4-6, 6-1. Ivanisevic came close to departing the court sooner, his hot temperament taking him to within a point of disqualification.
Cautioned first for ball abuse after Rafter broke for 2-0 in the fourth set with a running forehand down the line, Ivanisevic then received a point penalty for racket abuse in the fifth game. Rafter knew the end was nigh.
"I have to accept that he was the better player today," Ivanisevic said. "I lost my cool so many times. I was a little stupid. I was hitting the same stupid serves all the time and made the same stupid mistakes. One of my mishits was like a home run."
In the quarter-finals, Rafter will play Sweden's Jonas Bjorkman, who yesterday eliminated the Dutchman Jan Siemerink, Rusedski's third round conqueror, 6-4, 2-6, 6-2, 6-2.
Sweden's Thomas Johansson, who reached the fourth round due to Richard Krajicek's retirement, advanced to play Philippoussis in the quarter-finals, defeating the Russian, Yevgeny Kafelnikov, the No 11 seed, 3-6, 6-3, 6- 3, 7-6.
A second Swede advanced unseeded to the last eight, Magnus Larsson beating Germany's Oliver Gross, 6-4, 7-5, 5-7, 6-2.
Jana Novotna, the Wimbledon champion, reached the women's singles semi- finals here for the second time with a 6-2, 6-3 win against Patty Schnyder, the Swiss No 11 seed who eliminated Steffi Graf in the fourth round. Novotna lost to Graf in her previous semi-final, in 1994.