(CNN) -- Novak Djokovic faces a race against time to be fit for this week's Monte Carlo Masters after revealing he is struggling to overcome an ankle injury.
The World No.1 suffered the blow after rolling his right ankle during Serbia's Davis Cup victory over the U.S. last week.
The 25-year-old, who is based in Monte Carlo, is keen to compete in the tournament and will make a decision on Tuesday after being given a first round bye along with the other top seven players.
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"I can't guarantee 100 percent that I'll be on court. I have to be realistic and cautious," he told reporters.
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"I've had three days of practice and I've been increasing the level each day.
"If I have discomfort over the next two days, that would be an indication if I should or should not play. This is a strong event and there is no room for compromise.
"This is the start of a new season on a new surface. To compete at a high level I must be 100% fit."
With the clay court season about to kick off, Djokovic is keen not to take any unnecessary risks ahead of the Masters events in Madrid and Rome before going into the French Open next month.
The six-times grand slam winner added: "I've been taping the ankle and doing all I can to be ready on the court.
"I know there is a lot of expectation for me to play - no one wants me to be out there more than I do. I consider this a very special place and I'm extra motivated.
"I'm really encouraged by the progress over the last few days, it's much more than I would have expected it to be. I'll do everything in my power to play."
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Elsewhere, John Isner won his career title after overcoming Spain's Nicolas Almagro in the final of the U.S. Clay Court Championship.
The home favourite, ranked 23rd in the world, picked up the $82,000 prize following a 6-3 7-5 victory.
Meanwhile, Spain's Tommy Robredo claimed his first title in two years after defeating South Africa's Kevin Anderson 7-6 4-6 6-3 in the final of the Grand Prix Hassan II in Morocco.
Robredo, 30, had not won a title since triumphing at Santiago in February 2011, but will now return to the top 50 following his success.
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Venus Williams was forced to withdraw from the tournament after complaining of a lower back injury. The three-time Miami winner, who was set to face fellow American Sloane Stephens in the third round, hopes to return to action next month.
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The former World No.5 missed five months of 2012 with a leg injury but has begun to fight back after his ranking had plummeted to 471.
"I'm very happy to win a title again after my injury," he told reporters.
"Those points will help me a lot in the ranking. The goal is to arrive in Roland Garros in the best form possible. After that I will sit down and have a look at my goals."
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In Poland, Italy's Robert Vinci won the eighth singles title of her career after defeating Petra Kvitova 7-6 6-1 in the final of the Katowice Open.
The win will propel the Italian up to 12th in the world -- her highest ever ranking.
"I played a really good game today," Vinci told reporters.
"Petra was playing very well and very aggressively in the first set, but I stayed with her and won the tiebreak, and things were better in the second set, probably because she was a little bit tired."
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World No.9 Caroline Wozniacki suffered a shock defeat by Spain's Garbine Muguruza -- a woman ranked 64 places below the Dane. Muguruza needed just 81 minutes to claim a 6-2 6-4 win and leave her opponent stunned.
Agnieszka Radwanska edged through to the fourth round after seeing off Magadalena Rybarikova in three sets. Radwanska, the defending champion, won the first set 7-5 before losing the second 6-2. But the Pole, ranked fourth in the world, hit back to take the decider 6-3. She will face Sloane Stephens in the next round.
Li Na continued her comeback from injury with a 6-2 6-4 win over American Varvara Lepchenko. The Chinese star, who missed seven weeks of action after sustaining an ankle problem during her Australian Open final defeat, will face Spain's Garbine Murgurza in the next round.
Australian Open champion Victoria Azarenka was forced to withdraw from the tournament with an ankle injury. "It was really obvious what the decision was couple days ago already," Azarenka told reporters. I just wanted to give my 100 percent possibility to play and today was my last test."
Maria Sharapova will be hoping to progress to the fourth round Sunday when she takes on fellow Russian Russian Elena Vesnina. Sharapova, who won at Indian Wells last weekend, eased past Canada's Eugenie Bouchard 6-2 6-0 Friday following a power outage.
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