(CNN) When Kei Nishikori and Stan Wawrinka met in the US Open semifinals, the former led by a set and was in control in the early stages of the second. Then Wawrinka rallied -- and the Swiss went on to win a third grand slam title.
On Monday at the ATP World Tour Finals in London, the Japanese baseliner didn't let his lead slip. Indeed he thumped a listless Wawrinka 6-2 6-3 in 67 minutes to open play in the John McEnroe group.
"I felt very comfortable out there," Nishikori told reporters. "Started very good, from the first game. So I was very confident.
"I saw that he wasn't playing his best and he was missing. A lot of unforced errors. I saw a lot of opportunity today so I tried to be aggressive."
Read: Wawrinka wins US Open
Later Monday in the same group, Andy Murray plays his first match as the official No. 1 when he faces Croatia's Marin Cilic. Murray, a Scot who resides near London, is sure to have the crowd behind him at the O2 Arena.
Nishikori is one of only two men to defeat the surging Murray since the start of September -- Juan Martin del Potro being the other. He is a flashy shot-maker who Roger Federer, for one, said he likes to watch.
Eight of the best
Murray has the highest win percentage on the tour this year and, having followed in the footsteps of Tim Henman (2003) and Greg Rudedski (1998) with victory at the Paris Masters earlier in November, he's now attempting to become the first Brit in history to win the ATP World Tour Finals. A "Group of Death" stands in his way -- with world No.3 Stan Wawrinka having won seven of his 16 matches against Murray in his career. But, in the midst of a 19-match winning streak, Murray won't want to concede the world No.1 spot any time soon.
• Titles in 2016: 8 - Wimbledon, Olympics, Queen's Club, Paris Masters, Rome Masters, China Open, Shanghai Masters, Vienna Open
• Aces in 2016: 527
• Win percentage in 2016: 89% -- highest of the eight contenders
Facing world No.1s in major finals, Wawrinka has never lost -- beating Rafa Nadal in Melbourne (2014), Djokovic at Roland Garros (2015) and the Serb once again at this year's US Open. A man for the big occasion, Wawrinka holds a 100% record against Djokovic in grand slam deciders, but has never otherwise beaten him (0-19). With the US Open title to his name, Wawrinka became the oldest Grand Slam champion (31) since Andre Agassi at the 2003 Australian Open. A remarkable run of winning 11 straight finals only came to an end when he suffered a shock loss in St Petersburg to Alexander Zverev -- citing him as the
"future of tennis." • Titles in 2016:
4 - US Open, Geneva Open, Dubai Championships, Chennai Open
• Aces in 2016:
436• Win percentage in 2016:
74%
The highest-ranked Asian player in ATP history, Kei Nishikori captured a fourth straight title at the Memphis Open in February before going on to win Olympic singles bronze in Rio, beating Nadal in three sets. Nishikori, 26, is the first Japanese singles medalist since Ichiya Kumagae at Antwerp 1920.
• Titles in 2016: 1 - Memphis Open
• Aces in 2016: 245
• Win percentage in 2016: 76%
Standing at 198cm, Cilic is the tallest of the ATP finalists and a full 20cm taller than opponent Nishikori. He reached a third consecutive Wimbledon quarterfinal in 2016, but lost out to Roger Federer despite leading by two sets and holding three match points. Capping a resurgent end to 2016, Cilic has led Croatia to the Davis Cup final with wins over Belgium, USA and France, and will take to the court against Del Potro's Argentina later in November. Murray partly has Cilic to thank for his No. 1 berth -- he upset Djokovic with a supreme display of power tennis in the Paris Masters quarterfinal, beating the Serbian for the first time in his 15th attempt.
• Titles in 2016: 2 Cincinnati Masters, Swiss Indoors Basel
• Aces in 2016: 636
• Win percentage in 2016: 69%
Djokovic became just the third man in history to hold all four majors at once when he won the French Open in June, and the first player to surpass $100 million in prize money. For now, his throne has been seized by Murray, but it would surely be unwise to write him off; Djokovic is bidding to capture his fifth successive ATP World Finals title, and would move ahead of Pete Sampras and Ivan Lendl in the overall list of winners, equaling Federer with a sixth title. The Serb has never lost to any of his opponents in the Ivan Lendl group -- Gael Monfils (13-0), Dominic Thiem (3-0) and Milos Raonic (7-0) -- and still retains a 71% career win percentage against Murray.
• Titles in 2016: 7 - Roland Garros, Australian Open, Canada Masters, Miami Masters, Madrid Masters, Indian Wells, Qatar Open
• Aces in 2016: 264
• Win percentage in 2016: 88%
Already the first Canadian man to break into the ATP top 10, Raonic became the first to reach a major singles final when he met Murray at Wimbledon. That coveted first Grand Slam title may have evaded him, but he did take the Brisbane International at the turn of the year. Raonic nailed a slam dunk at the 2016 NBA All-Star Celebrity Game, helping Canada beat USA 74-64. And his 6-feet 4-inch stature also helps him on the tennis court ... He wins 90.6% of service games -- the third highest percentage on the tour in the past year (after Ivo Karlovic and John Isner) -- and has the fastest serve (career-best 155.3 miles per hour) of all the ATP World Tour finalists. The Canadian, 25, has also saved an impressive 70.2% of break points -- the highest proportion of the eight.
• Titles in 2016: 1 - Brisbane International
• Win percentage in 2016: 77%
• Aces in 2016: 824
For so long looked upon as an entertainer rather than a genuine threat, Monfils is making his ATP World Tour Finals debut in 2016, having produced a career-best season in his 30th year. He reached his first Grand Slam semifinal for eight years at Flushing Meadow, and though he may have eventually lost out to Djokovic, he boasts a higher break point conversion percentage than any other player at the finals. It's been a promising year for the Frenchman; consistency allied with his undeniable talent could see Monfils finally hit the heights he's hinted at.
• Titles in 2016: 1 - Washington Citi Open
• Aces in 2016: 471
• Win percentage in 2016: 75%
The last man to qualify for this year's event, Thiem is the youngest player at the finals and the first Austrian to take part in the singles since former world No.1 Thomas Muster in 1997. He may have lost out to Djokovic at Roland Garros -- his first ever grand slam semifinal -- but boasts a 90.9% win percentage in deciding sets in 2016 -- the highest proportion of any player on the tour to have started 25 matches or more. Not bad at all for a 23-year-old.
• Aces in 2016: 462
• Win percentage in 2016: 72%
• Titles in 2016: 4 - Stuttgart's Mercedes Cup, Open de Nice Cote d'Azur, Acapulco's Abierto Mexicano Telcel, Argentina Open
But Nishikori didn't have to produce his best to see off Federer's fellow Swiss and good pal.
Wawrinka suffered from a sore knee recently and it likely played a part in his poor performance. He averaged nearly two unforced errors per game and his often breathtaking one-handed backhand severely misfired: He struck two backhand winners, making 15 unforced errors.
Yet the world No. 3 didn't apportion too much blame to the knee.
"I was feeling better on the court so I thought I could play at a better level today," Wawrinka told reporters. "I was expecting a good match. Didn't happen."
The fifth-seeded Nishikori broke in the fifth game of each set to take control. His break in the second set particularly stung Wawrinka, given the latter led 40-15.
It appeared Wawrinka might have awoken when he ripped a forehand winner down the line from outside the doubles alley in the eighth game of the second set but later in the game, he missed a second-serve return long and ballooned another second-serve return.
Wawrinka will take heart in knowing that he lost his first match last year, too, to Rafael Nadal at the World Tour Finals before making the semifinals. He has never failed to make the last four in his three previous appearances at the event.
But he can ill afford a repeat of Monday.
"I'm sure I still have something inside me to play some great tennis before the end of the year," said Wawrinka. "So I'm going to try everything for that in the next match. I'm going to do what I need to do tomorrow to get ready for trying to play better in two days."
Murray carries a 19-match winning streak into his clash with Cilic but lost to the 2014 US Open winner in Cincinnati in August.
The player he replaced atop the rankings, Novak Djokovic, began his tournament by beating Dominic Thiem on Sunday. Djokovic -- the four-time defending champion -- is assured of finishing the year as No. 1 if he wins one more round-robin match and then the title.