Can Nadal stay on top in 2020?
Rafael Nadal arrived at the Italian Open in May without a title in 2019. He beat Novak Djokovic in the final and lost just three matches in the rest of the season, winning the French Open and the US Open before inspiring Spain to Davis Cup glory on home soil, finishing as world number one for a fifth time.
With a combined age of 103, Nadal, Djokovic and Roger Federer continue to dominate men's tennis and it takes a brave punter to predict their imminent demise. But there is at last the sense of a credible challenge from an exciting crop of youngsters including Daniil Medvedev, who pushed Nadal all the way in the US Open final, and ATP Finals winner Stefanos Tsitsipas.
"I believe I'm really close to being crowned a Grand Slam champion," said Tsitsipas after beating Dominic Thiem in the final in London.
Women's NextGen show the way
Australia's Ashleigh Barty, 23, rose from the relative anonymity of 15th in the world at the end of 2018 to top the rankings a year later, winning the French Open and the WTA Finals along the way.
Teenager Bianca Andreescu of Canada soared even more dramatically, from 178th to fifth in the world, beating Serena Williams in the final of the US Open to become the first Canadian to win a Grand Slam singles title. "NextGen is here," Andreescu announced during her winning run at Flushing Meadows.
"You see many fresh faces and I think we're only going to get better from here."
Federer's Wimbledon pain
Roger Federer had two Wimbledon championship points on his own serve to beat Novak Djokovic and win his 21st Grand Slam title. That would have taken him six clear of the Serbian on the all-time list and three clear of Rafael Nadal at the time. Instead, he faltered.
Djokovic, in a match lasting a shade under five hours, won his 16th Grand Slam and two months later Nadal won his 19th.
"It's such an incredible opportunity missed," said the Swiss veteran, now 38 and who must fear his days as the all-time leader on the list are numbered.
Serena's Court problem
Serena Williams is running out of opportunities to win the 24th Grand Slam singles title that would pull her level with Margaret Court's record. The 38-year-old American reached the final at both Wimbledon and the US Open but she won just four games against Simona Halep in London and lost in straight sets to Bianca Andreescu in New York.
It followed defeats in the finals of both events the previous year, suggesting Williams is feeling the pressure. Speaking after her US Open defeat, she downplayed the pursuit of Court's record. "I'm not necessarily chasing a record. I'm just trying to win Grand Slams," she said.
"It's definitely frustrating, but for the most part I just am still here. I'm still doing what I can do."
Comeback of the year
In January, British former world number one Andy Murray admitted the 2019 Australian Open could be his last tournament during a tearful press conference. But the Scot, 32, returned to the court after hip resurfacing surgery and beat fellow veteran Stan Wawrinka in the final of the European Open in October.
"It means a lot," Murray said after his triumph.
"The past few years have been extremely difficult.... I didn't expect to be in this position at all, so I'm very happy."
A star is born
Having been the youngest player to come through Wimbledon qualifying, 15-year-old Coco Gauff stunned five-time champion Venus Williams in the first round in straight sets, belying her ranking of 313.
"My dream was to win. That's what happened," said the American whose run at the All England Club was eventually ended by champion Simona Halep in the fourth round.
Gauff went on to make the US Open third round before winning a maiden title in Linz in the autumn having lost in qualifying. At the end of 2018, she was ranked 875 but she will head into 2020 at 68.