Austria's Stefan Koubek toppled French Open champion Gaston Gaudio of Argentina 6-1, 1-6, 6-3 on Thursday to reach the quarterfinals of the $US1.25 million ($A1.68 million) Swiss Indoors.

Later, second-seeded Tim Henman struggled to overcome South African Wesley Moodie, converting only one of 12 break points to win 7-6 (2), 6-7 (0), 6-4 and reach the quarter-finals for the eighth consecutive time in Basel.

"It was closer than I would have liked it to be," said Henman, who's been taking supplements for a magnesium deficiency that fatigued him in Madrid last week.

"It was a long match and it's a good sign that I'm feeling in pretty good shape. The tell-tale sign will be how I feel tomorrow morning."

Fifth-seeded Tommy Robredo withdrew from the tournament with a right shoulder injury, sending Czech Jiri Novak into the quarter-finals, while fourth-seeded David Nalbandian, the 2002 champion, downed American Vincent Spadea 6-4, 6-4.

It was a second straight upset for Koubek, listed 64 spots below the ninth-ranked Gaudio. The Austrian began the tournament with an impressive win over the 25th-ranked Paradorn Srichaphan. It also marks the first time he's reached the quarter-finals of an international series tournament since September 2003.

Henman, who crashed out in the third round of last week's Madrid Masters because of his mineral deficiency, is still looking to secure his berth in the Tennis Masters Cup in Houston on November 15.

The Briton was aced 24 times by Moodie, and squandered four break points in the opening set, yet managed to tame the South African in the tiebreak. Undeterred, Moodie continued to trouble Henman in the second. A combination of unforced errors by the Englishman and solid play by Moodie in the second tiebreak left the pair at a set apiece.

Henman finally prevailed, converting the third of three match points for the first break of the contest.

"It's tough when you've got a guy serving that big and that consistently," Henman said.

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