- Novak Djokovic reveals that his father's condition is steadily improving
- Srdjan Djokovic was hospitalized at the start of last week before son lost in Paris
- Serbian has bounced back on court this week in London, reaching ATP semifinals
- U.S. Open champion Andy Murray also through to last four in season finale
Novak Djokovic has a smile on his face again -- and not just because of his perfect record at this week's ATP World Tour Finals.
At the start of last week, the Serbian tennis star was battling with the news that his father had been taken seriously ill.
It badly affected his preparations for the Paris Masters, and he lost his opening match.
Srdjan Djokovic was hospitalized with a reported acute respiratory condition, but his condition has since improved -- which has allowed his son to find focus for the $5.5 million season-ending event in London.
"Well, it was hard. It still is, but there is good news every day," said the world No. 1, who lost his grandfather in April just hours before playing a match in Monte Carlo.
"That makes me happier, makes me play more relaxed on the court. I switch off on the tennis court. I try to think only about the match and it's been working well so far.
"Every win means a lot for my family and me. But there are more important things in life, and that's health. I guess that's the priority now."
Djokovic is through to the semifinals of the elite eight-man tournament following Friday's 6-2 7-6 (8-6) victory over sixth-ranked Czech Tomas Berdych, which gave him a 100% record in three Group A matches.
As he did in Wednesday's win over world No. 3 Andy Murray, Djokovic was forced to battle hard and he saved three set-points to prevent the match going to a decider.
"I played a really good match but I thought he served better towards the end of the second set. I was fortunate to come back and win in straight sets," said Djokovic, who ended Berdych's hopes of repeating last year's progress to the semifinals with his 11th win in their last 12 encounters.
It took him to 73 match victories this year, equaling the leading mark set by David Ferrer.
Djokovic won the tournament in 2008 its final staging in Shanghai, but last year failed to qualify out of a group including Berdych.
He ran out of steam following a breakthrough season in which he toppled Rafael Nadal from the No. 1 ranking and claimed three of the four grand slam titles.
Berdych will now look forward to next week's 100th Davis Cup final, where he will lead his country against Spain.
Murray also made it through to the last four in London, beating Frenchman Jo-Wilfried Tsonga 6-2 7-6 (7-3).
The U.S. Open and Olympic champion needed only to win the first set to go through, following Berdych's defeat, and did so with few problems before Tsonga made a match of it.
Murray and Djokovic must wait for Saturday's Group B results before finding out who they will play on Sunday.
Wimbledon champion Roger Federer has already secured a semifinal place, but the result of his match against Juan Martin del Potro will decide who tops the group.
World No. 5 Ferrer -- who has a tour-leading seven titles this year -- can also go through if he beats Janko Tipsarevic and defending Federer wins his third successive match.