The hard work continued for top seeds Pete Sampras and Michael Chang as they progressed battered, but intact, into the semi-finals of the US$328,000 Salem Hong Kong Open last night. Both players were taken to three sets at Victoria Park, number one seed Sampras taking 127 minutes to beat German David Prinosil 6-7 (2-7), 7-6 (7-3), 6-3 in his quarter-final. Second-seeded Chang was stunned by a transformed Czech Martin Damm in the second set as he struggled to a 6-0, 3-6, 6-3 triumph. They were joined in the last four by fifth-seeded Dutchman Jan Siemerink, who faces Sampras, and Australia's sixth seed Todd Woodbridge, who knocked out 1991 Hong Kong champion Richard Krajicek, of Holland, 6-4, 7-6 (7-5). Sampras, who had a tough two-setter against Australian Patrick Rafter in the previous round, said he found it difficult to assert his authority on Prinosil. 'But I felt I raised my level when I had to and dug myself out of a hole.' Chang cruised in the first set against Damm, but from then on had a battle on his hands. 'I think he got off to a slow start but in the second set, he started to play much better and I might have relaxed just a little bit,' said Chang, aiming for his third consecutive title in Hong Kong. 'The third set was definitely a struggle.' Chang, who has had to win three tough matches to reach the semi-finals, expects another hard time against Woodbridge today. 'Todd is a tough customer,' he said. 'He's definitely an accomplished player and people don't know that he's not only a good doubles player.' Ironically, soon after his singles victory over Krajicek, Woodbridge and his world-beating doubles partner Mark Woodforde were dumped out of the doubles competition, 6-3, 6-3 by Americans Kent Kinnear and Dave Randall. Siemerink advanced to the semi-finals with a 6-3, 2-6, 7-6 (7-2) victory over Japan's Shuzo Matsuoka.