MADRID (AFP) - Novak Djokovic equalled Ivan Lendl's 1986 record of a 29-0 winning start to a season on Thursday when he reached the Madrid Masters quarter-finals. The Serb second seed cruised to a 6-1, 6-2 thrashing of Spaniard Guillermo Garcia-Lopez which took less than an hour to extend his overall streak to 31, stretching back to the two Davis Cup final singles he won in December. The record for a winning start to a season remains 42 matches, set by John McEnroe in 1984, with Bjorn Borg, who had 33 straight wins in 1980, in second spot. Top-seeded holder Rafael Nadal went through to the last eight without lifting a racquet as opponent Juan Martin Del Potro retired before their third round match with hip pain. Unseeded Del Potro, the 2009 US Open champion, who is making a recovery this season after missing more than eight months of play in 2010 after wrist surgery, had been bothered by the problem since reaching the Estoril semi-finals at the weekend. "I have experience with injuries, and I decided not to play," Del Potro said as he prepared to fly home to Buenos Aires for medical checks. The Argentine will not play the Rome Masters starting on Sunday and could well be in doubt with the French Open starting on May 22. "The pain was very strong," said the Argentine. Third seed Roger Federer got past Belgian Xavier Malisse 6-4, 6-3 after a late-night struggle on Wednesday against Feliciano Lopez which went three hours. "It was a quick turnaround," said the two-time champion. "Mentally it was tough, I had to be ready to play a different kind of match (from the night before). "I didn't have high expectations, I just wanted to play a good match. I was able to do that." In the women's event, world number one Caroline Wozniacki lost to Germany's Julia Goerges for the second time in little more than a week while French Open champion Francesca Schiavone and Roland Garros runner-up Samantha Stosur also went out. Goerges, who saw off Wozniacki in the Stuttgart final last week, enjoyed a 6-4, 1-6, 6-3 win and next plays Russian Anastasia Pavlyuchenkova, who upset Stosur 7-6 (7/4), 6-3. "I'm quite proud of the last three weeks, I was able to play more aggressively today. I didn't give her any rhythm," said Goerges, the world number 27. "I've been working hard on my fitness over the last 18 months and I think it's starting to show results." America's Bethanie Mattek-Sands defeated third seed Schiavone 7-6 (7/5), 6-3 but there were no problems for fourth seed Victoria Azarenka who overcame Spain's Arantxa Parra Santonja 6-0, 6-3. Sixth seed Li Na of China, the Australian Open finalist, continued her progress, moving into the last eight thanks to a 2-6, 6-2, 6-1 win over Italian Roberta Vinci, who won in Barcelona last weekend.