(CNN) -- Frenchman Jo-Wilfried Tsonga is rated as doubtful for next week's Australian Open -- where he is seeded fifth -- after withdrawing from his Sydney International quarterfinal with a back injury, reports the tournament's official Web site.
Tsonga, who reached last year's final at Melbourne Park where he lost to Novak Djokovic, was scheduled to play Finland's Jarkko Nieminen on Thursday.
However, the world number six was bothered by his back during his opening match with Simone Bolleli, receiving treatment during the breaks.
Meanwhile, top seed Djokovic won his rain-interrupted quarterfinal against Croatia's Mario Ancic 6-3 6-4 in a match that lasted over three-and-a-half hours, because of two rain breaks.
The first break came at 1-1 in the second set and lasted for 20 minutes, whilst a longer delay of one hour and 25 minutes occurred a short time later when Djokovic was up a break at 3-1 in the second set.
Meanwhile, Lleyton Hewitt's hopes of a fifth Sydney title were ended by fourth seed David Nalbandian, who won a close encounter 7-6 7-5.
Nalbandian will now face Richard Gasquet in Friday's semifinals, after the Frenchman saw off compatriot Jeremy Chardy 6-2 7-6.