Joachim Johansson faces an anxious wait to discover if he will be fit for next week's Australian Open after a hamstring injury forced him out of the Sydney International on Wednesday.
The big-serving Swede was trailing Max Mirnyi of Belarus 4-1 when he had to retire from their second round match, leaving Australia's Lleyton Hewitt as the clear favourite to win the tournament.
Hewitt, who has won the Australian Open warm-up event three times including last year, eased his way into the quarter-finals with a 6-2, 6-4 win over Frenchman Arnaud Clement.
Third seed Johansson told a news conference he had initially pulled the muscle in his first round win over Briton Greg Rusedski on Tuesday and did not want to risk further damage against Mirnyi.
"I tried to go out there for a few games but I felt I couldn't give 100 percent, and at this level there's no way you can win," he said.
"If it had felt good I would have kept playing but obviously it doesn't so that's why I stopped."
Johansson was considered one of the main title contenders for the first Grand Slam of the season after finishing last year ranked 11th in the world and winning last week's Australian hardcourt championship in Adelaide.
FULL RECOVERY
He said he planned to fly to Melbourne on Wednesday night to see a specialist in the hope of making a full recovery in time for the Australian Open which starts on Monday.
"It's too early to say," he said. "At the moment it doesn't feel great but hopefully I'll be back practising at the end of this week."
Johansson was the latest high-profile casualty at the Australian Open warm-up event at Sydney's Olympic tennis centre.
Former world number one Carlos Moya of Spain was knocked out by Czech Radek Stepanek on Tuesday, two days after winning the Chennai Open, while women's French Open champion Anastasia Myskina of Russia went out at the hands of Chinese qualifier Shuai Peng.
Another Russian, fourth seed Vera Zvonareva, went out on Wednesday after a 7-6, 6-3 loss to Fabiola Zuluaga of Colombia.
Hewitt raced through the first set against Clement but trailed 4-1 in the second set before winning the last five games to extend his winning sequence in Sydney to 17 matches.
"Momentum changes so much on days like today when it's so hot out there," Hewitt said.
"Once you start getting down on yourself a little bit it can snowball but when you're positive it's a lot easier to play good tennis and win a few games in a row."
Hewitt will play Thomas Johansson in the quarter-finals after the Swede beat lanky Australian Wayne Arthurs in two tiebreakers with big-serving American Taylor Dent to play Mirnyi.
Romania's fourth seed Andrei Pavel will play Czech qualifier Ivo Minar and Stepanek faces Spanish fifth seed Feliciano Lopez after they all won.
Elena Dementieva, last year's French and U.S. Open finalist, beat Greek Eleni Daniilidou 6-3, 3-6, 6-2 while fellow Russian Nadia Petrova thrashed Daniela Hantuchova 6-0, 6-2.
Switzerland's Patty Schnyder and Australia's Samantha Stosur, the finalists in last week's Australian hardcourt final, also advanced along with Australia's Alica Molik. World number one Lindsay Davenport did not play on Wednesday.
