Nalbandian complained of muscle spasms in his back in a hitting session at Melbourne Park, the Australian Open venue.

The 26-year-old Argentine player was advised to take two to three days off and he would assess his condition this weekend before deciding whether he would play next week at the season's first major, beginning Jan. 14.

Top-ranked Federer withdrew from the Kooyong event on Monday, citing a "serious stomach virus," but was back in practice today at Rod Laver Arena, the center court at Melbourne Park where he has won three of the past four Australian Open titles.

He moved freely and showed no signs of discomfort in a short hitting session after exercising on the new, blue Plexicushion courts.

Federer has previously used Kooyong — an invitation-only event stacked with top players attracted by guaranteed matches — to fine tune his Australian Open campaigns.

Tournament director Colin Stubs said there was a possibility of Federer having a match later this week should a vacancy become available.

Nalbandian was replaced in the field by young Australian Brydon Klein, but Federer could be brought back if there was a further withdrawal.

"He said he would like the match practice," Stubs said.

"We can slot a match outside the format. Federer, (James) Blake, (Novak) Djokovic are not out of the equation for a fourth match outside the bracket."

Kooyong has installed the same Plexicushion surface that the Australian Open is using this year for the first time, so a chance on the new surface in match conditions would suit Federer, who had declared himself fit to defend his Australian Open title.

"I hung around and I definitely think it's going to turn for the good and I'll be 100 per cent, really, before the (Australian Open) starts," he said at a sponsor's promotion late Tuesday.

He would ask for a Tuesday start at the Australian Open, although as defending champion and No. 1 seed, he would normally be allocated a Monday slot.

Marcos Baghdatis, who lost the 2006 final to Federer, replaced the Swiss star at Kooyong and opened with a 6-4, 4-6, 6-1 win over No. 4-ranked Nikolay Davydenko of Russia today.

Andy Roddick, seeking a third consecutive Kooyong title, had a 6-3, 6-0 win over Ivan Ljubicic and Safin beat Andy Murray 6-1, 6-4 in a match brought forward because of Nalbandian's withdrawal.

Fernando Gonzalez, who made last year's Australian Open final, received a walkover over Nalbandian into the next stage of the progression-relegation event. He won a warmup match 7-6 (3), 6-4 against Klein for match practice.