Novak Djokovic begins his bid for a sixth Wimbledon title and 20th Grand Slam on Monday as the grass court showpiece returns following its cancellation in 2020 due to the COVID-19 pandemic.

The world No.1 overcame an early scare to reach the second round of Wimbledon with a 4-6, 6-1, 6-2, 6-2 win over British teenager Jack Draper.

The 34-year-old struggled on the slippery Centre Court surface with the roof closed above it due to rain.

Left-handed Draper, playing just his fifth match on the main tour, saved seven of seven break points in the opener as he stunned the top seed.

But Djokovic soon snuffed out any danger of him becoming only the third defending champion to lose in the first round, by sweeping through the remainder of the tie.

He finished with an impressive 25 aces and 47 winners as his 19-year-old opponent wilted.

Having already captured the Australian and French Open titles this year, Djokovic is halfway to becoming just the third man in history to complete a calendar Grand Slam.

It's a feat so rare that Rod Laver was the last man to achieve the sweep back in 1969.

In what could be a record-breaking year for Djokovic, there is also the chance of an Olympic title, paving the way for a Golden Slam of all four majors and Tokyo gold.

"Wimbledon, Olympics and U.S. Open, it will be fun to watch Novak play because he's going to put everything on himself to try to make it," said world No.2 Daniil Medvedev.

"That's when he's strongest, but at the same time that's a lot of pressure."

American Frances Tiafoe caused the first big upset of the Wimbledon men's singles tournament by beating third seed Stefanos Tsitsipas 6-4 6-4 6-3 on Monday.

Tsitsipas, playing his first match since his French Open final defeat by Novak Djokovic on June 13, was on the back foot from the beginning against a 57th-ranked opponent who took the game to him.

The Greek also went out in the opening round on his last previous appearance at the grass court tournament in 2019, when he was seventh seed.

"It's definitely one of my best (matches). From start to finish it was pretty clean," Tiafoe told the cheering 6,000 strong Court One crowd, sheltered by a roof from the drizzle outside, as the result was read out.

Also, American Sloane Stephens won the battle against twice Wimbledon champion Petra Kvitova ousting the Czech 10th seed from the All England Club with a 6-3 6-4 victory on Monday.

The 2017 U.S. Open champion came into the match on Centre Court with a 2-1 head-to-head lead against Kvitova, who lifted the Venus Rosewater Dish in 2011 and 2014, but all their previous meetings had been on hard courts.

(With input from agencies)