As the only one of the four grand slam titles he has yet to win, Novak Djokovic needs no extra incentive to clinch victory at the French Open here this weekend.

Yet having learned of the death of his first coach, Jelena Gencic, who the world No.1 said was like "a second mother" at the weekend, the Serb said he was more determined than ever to complete the full set of grand slam crowns.

Yesterday, after beating Germany's Philipp Kohlschreiber 4-6, 6-3, 6-4, 6-4 to reach the quarter-finals, Djokovic spoke warmly, maturely and touchingly about the impact Gencic had on him. "It hasn't been easy, but this is life," he said. "Life gives you things, takes away close people in your life and Jelena was my first coach, like my second mother. We were very close throughout my whole life and she taught me a lot of things that are part of me, part of my character today. I have the nicest memories of her."

Gencic, who died from a heart attack on Saturday, played both tennis and handball for the former Yugoslavia before becoming famous as the woman who discovered Monica Seles and Goran Ivanisevic. She coached Djokovic from the ages of six to 12 and together with providing the base of the game he plays today, she taught him about music, literature and life.

Unable to attend a service for Gencic in Belgrade yesterday, Djokovic sent a heartfelt letter in which he described his coach as an angel. Expressing his sadness that he could not say goodbye, he said he knew she would be mad if he did not try to play on and fulfil their dream of winning the title here.

"I feel even more responsible now to go all the way in this tournament," he said after his win over Kohl-schreiber. "I want to do it for her because she was a very special person in my life. I remember the last conversation we had two weeks ago about Roland Garros . . . she told me, 'listen, you have to focus, you have to give your attention to this tournament'. This is a tournament you need to win'.

"So now I feel in her honour that I need to go all the way. but it's not about me only. There are so many great players around still in the tournament. But it gives me that inner strength to push even harder."

Djokovic will next play the 35-year-old German Tommy Haas, who crushed Mikhail Youzhny in straight sets but, if he is to win the title, he knows he will probably have to beat Rafael Nadal in the semi-finals and the seven-times champion yesterday moved up a gear in a 6-4, 6-1, 6-3 victory over Kei Nishikori, of Japan.

Having dropped a set in his opening two matches and struggled past Fabio Fognini in round three, Nadal looked more his usual self as he celebrated his 27th birthday with a comfortable win, after which tournament organisers presented him with a massive, very yellow, cake.

"I played much better today than the first three matches, no doubt about that," said Nadal, who revealed that soreness in an elbow, which he said he felt in Rome last month, was no longer a problem.

"I started to feel my forehand much better and I started to change directions well. I started to feel that I can hit the ball longer and having more confidence in every stroke." Facing him is appearing an increasingly daunting prospect.

His quarter-final opponent will be Stanislas Wawrinka, the Swiss No.2 who came from two sets down to beat Frenchman Richard Gasquet 6-7, 4-6, 6-4, 7-5, 8-6 in a match full of spectacular shot-making. The fourth set was almost flawless and in four hours and 16 minutes, Wawrinka had smashed 92 winners of his own.

On the women's side, defending champion Maria Sharapova will take on the former world No.1 Jelena Jankovic after both saw off young Americans. No.2 seed Sharapova beat Sloane Stephens 6-3, 6-4 to reach the quarter-finals while Jankovic crushed Jamie Hampton 6-0, 6-2. The third-seeded Belarussian Victoria Azarenka and No.12 Maria Kirilenko also advanced to the last eight.

Today, Roger Federer plays the last Frenchman standing, Jo-Wilfried Tsonga, while women's favourite Serena Williams takes on former champion Svetlana Kuznetsova.