Novak Djokovic was gifted a place in the Wimbledon second round as injured Martin Klizan was forced to retire/

Djokovic, a three-time Wimbledon champion, was leading 6 -3, 2-0 when Slovakian World No.47 Klizan retired after just 40 minutes on Centre Court due to a calf problem.

The 30-year-old Serb will face Adam Pavlasek of the Czech Republic for a place in the last 32.

"He had issues walking on to court. I tried to focus on my game plan, I was serving well and when it mattered I made a break," said Djokovic, who is aiming to win a first Grand Slam title since last year's French Open.

"It was great to be back on Centre Court. It's the cradle of tennis history and it's a special feeling to walk on, but you never like to end a match this way."

It was the kind of trouble-free first round Djokovic would have been hoping for after the most turbulent period of his career.

The World No.4 crashed out in the third round at Wimbledon in 2016 and has struggled badly in 2017, the 12-time major winner losing in the Australian Open second round and the French Open quarter-finals.

Milos Raonic, the men's runner-up in 2016, advanced to the second round with a 7-6 (7/5), 6-2, 7-6 (7/4) win against Jan-Lennard Struff.

Big-serving Raonic became the first Canadian man to make a Grand Slam final with his impressive Wimbledon run 12 months ago before losing the title match to Andy Murray.

The sixth seed made a business-like start to his bid to make a second major final, in the process extended German world number 53 Struff's losing streak at Grand Slams to 10 matches.

French 22nd seed Richard Gasquet slumped to a 6-3, 6-4, 5-7, 6-2 loss against Spain's David Ferrer.

It was the first time Gasquet, a two-time Wimbledon semi-finalist, had lost in the first round since 2006.

Argentina's Juan Martin Del Potro, a semi-finalist in 2013, defeated Australian Thanasi Kokkinakis 6-3, 3-6, 7-6 (7/2), 6-4 as the 29th seed moved towards a potential third round classic against Djokovic.

Australia's Bernard Tomic said he had lost his "respect" for tennis following his lacklustre 6-4, 6-3, 6-4 loss to Mischa Zverev.

"I wasn't there with my mental state to perform," Tomic said.

"I don't know why but I felt a little bit bored out there, to be completely honest."

Gael Monfils, the flamboyant French 15th seed, crushed German qualifier Daniel Brands 6-3, 7-5, 6-4.

In the women's draw, former French Open champion Garbine Muguruza, the 14th seed, defeated Russia's Ekaterina Alexandrova 6-2, 6-4.

Australian qualifier Arina Rodionova, ranked 166th, saved seven match points before defeating Russian 16th seed Anastasia Pavlyuchenkova 3-6, 7-6 (8/6), 9-7.

Later on Tuesday, Roger Federer, chasing a record eighth Wimbledon title, starts his 19th All England Club campaign against Ukrainian World No.84 Alexandr Dolgopolov.

Having unexpectedly won the Australian Open in January before taking the clay-court season off to rest, Federer can lay down a marker to his title rivals by claiming a record 85th match win at Wimbledon.