When the 2020 men's tennis season ended at the ATP Finals, Rafael Nadal captured a common sentiment that still holds true as the 2021 tennis season is set to begin this week.

"The world is suffering a lot, so we are very lucky that we can play tennis," Nadal said in London in November. "That's the real feeling that I have."

After an off-and-on, pandemic-altered year for the sport, coronavirus questions and an ever-evolving calendar still will be key parts of the conversation as things get going Wednesday at the Abu Dhabi WTA Women's Tennis Open, where Sofia Kenin leads the field, and Thursday at the ATP's Delray Beach Open, where Reilly Opelka is the defending champion.

Opelka won that hard-court title last February, but the tournament was moved to early January as part of a shuffling on both professional tours.

"It's really going to be for me to find my tennis. I haven't even played in three months," said Opelka, a 23-year-old American who is based in Florida and ranked 39th. "So this is really just to get me back into the competitive mindset and playing against other guys and back in tournament mode."

The changes include the start of the year's first major championship, the Australian Open, delayed three weeks to Feb. 8, and the joint event played at Indian Wells, California — canceled last year because of the COVID-19 outbreak — giving up its usual March dates without a new spot for it on the schedule as of now.

There are some significant milestones in the offing for stars such as Nadal, Novak Djokovic and Serena Williams in 2021.

Nadal can break a tie with Roger Federer for the most Grand Slam singles titles for a man after matching him with No. 20 at the French Open in October; by March, Djokovic can surpass Federer's record for most weeks at No. 1 in the ATP rankings; Williams is one Slam singles trophy away from No. 24, which would equal Margaret Court for the most in the sport's history.

Other tennis storylines in 2021:

FEDERER'S RETURN

More than 12 months will have passed between matches for the 39-year-old Federer by the time he returns from two operations on his right knee. Federer's agent told The Associated Press he is aiming for late February after missing the Australian Open for the first time since his main-draw debut in 2000.

THE SLAMS

There were only three Slams in 2020, because the All England Club canceled Wimbledon for the first time since World War II. As of now, Wimbledon is scheduled for its usual place on the calendar, beginning June 28. The French Open's start got shifted from May to September last year and the number of fans was severely restricted; as of now, it is slotted for May 23. For the Australian Open, there are lingering questions about flights, hotels and quarantine plans for players and their personnel. "I'm not that excited, to be honest. Like, I don't want to stay in my room for 19 hours," Opelka said. "You know, I get it; we have to do it. But I'm not one bit excited."

TOKYO OLYMPICS

The 2020 Olympics, as they'll still be called, are scheduled to start in July, and two tennis players representing Japan could be breakout stars: AP Female Athlete of the Year Naomi Osaka  and Kei Nishikori. "The Olympics are a special time when the world comes together to celebrate sports. I am looking forward most to being with the athletes that had waited and trained for over 10 years, for celebrating a very hard year (2020), and having that happen in Japan makes it that much more special," Osaka wrote in an email interview. "It's a special and beautiful country filled with culture, history and beauty. I cannot be more excited."

CROWDED COMPETITION

Should be fascinating to watch the young women who recently won Grand Slam titles compete for more. That includes No. 1 Ash Barty, who is 24; No. 3 Osaka, 23; No. 4 Kenin, 21; No. 7 Bianca Andreescu, 20; No. 17 Iga Swiatek, 19.

WHO'S GOT NEXT?

There appear to be more legitimate candidates for first-time major champions in men's tennis than there have been in years. With Dominic Thiem joining the club last year at the no-spectator U.S. Open, who will join him? Daniil Medvedev, Andrey Rublev, Stefanos Tsitsipas and Alexander Zverev all appear capable.