American John Isner and world number nine Janko Tipsarevic are both in doubt for the Australian Open with injuries clouding their warm-up campaigns.
Isner admitted he may have to withdraw from the first grand slam of 2013 after he was troubled by a knee injury in a frustrating loss at the Sydney International.
Men's top seed Isner was beaten by compatriot Ryan Harrison 6-4, 6-4 in a second round match on Wednesday.
The world number 13 was feeling the effects of a knee problem picked up late last year and also admitted to being put off by the ATP's controversial new timing rules.
Isner came into the match with a wear and tear bone bruising injury on his right knee and, while it reacted better than expected, he feared he could make it worse by playing in Melbourne next week.
It means the big-serving American could opt out of the season's first grand slam and rest ahead of the US's opening-round Davis Cup clash with Brazil next month.
"I've got a decision to make shortly," Isner said.
"It was a lot better than it was six days ago but it wasn't 100 per cent. It needs rest is what it needs."
The 27 year-old said he would not be able to practice at the level required ahead of the Open and planned to make a call in the coming days.
"I could play (in Melbourne), you know. Certainly I could win a match maybe, a match or two," Isner said
"But I don't see myself winning the whole tournament at this point, that's for sure."
Isner became the latest in a string of players to criticise the ATP's recent crackdown strictly enforcing the amount of time players have between points.
He received a warning for breaching the 25-second limit during the first set and felt rushed throughout the match.
"You know, for someone who sweats a lot like me ... I felt like I could never go to the towel," Isner said.
"The umpire was (pointing at his watch) the whole match.
"I don't particularly like that rule, that's for sure, because it messed with my rhythm, my flow, and I'm sure it's doing that to a lot of other players, too."
Meanwhile Tipsarevic became the third player in 24 hours to pull out of the eight-man Kooyong Classic exhibition event with injury on Wednesday.
Just three days after winning the Chennai Open in India - and five days out from the Open - the Serbian quit the lead-in event on the opening day complaining of a wrist injury.
Tipsarevic was trailing Cypriot Marcos Baghdatis 1-6, 0-1 in his first match when he said he was unable to continue.
He said he was playing with the aid of painkillers when he scored his victory over Spaniard Roberto Bautista Agut in the Chennai Open final on Sunday.
The Kooyong tournament was battered by the withdrawal of star players as it got underway.
Japan's Kei Nishikori, ranked 18 in the world, pulled out when unable to shake off the knee injury which forced him to forfeit his Brisbane International semi-final spot last week.
He said he only began hitting again on Tuesday and quickly realised he would not be able to play at Kooyong.
The Japan Open winner was undergoing intensive treatment in a bid to be fit to play next week's Open and was replaced in the Kooyong Classic field by Croatia's world number 74 Ivan Dodig.
Juan Monaco, the Argentinian world number 12, was the first player to pull out.
Monaco told tournament director Colin Stubs on Tuesday that he would not be able to line up due to a hand injury.
He was uncertain how serious the problem was but doctors had advised him not to play in the next few days.
Monaco's spot in the eight-man field was taken by world number 58 Paul-Henri Mathieu of France who lost his first match to Juan Martin del Potro.
AAPTags: tennis, sport, melbourne-3000, australia, vic First posted January 09, 2013 18:30:33