As play began in the John McEnroe group on Monday, Kei Nishikori and Andy Murray earned victories at the ATP World Tour Finals in London.

Nishikori was impressive as he dismantled Stan Wawrinka 6-2, 6-3 in the first match. It was a rematch of their US Open Semifinal where the Swiss won a hard fought four-setter en route to his title at Flushing Meadows.

The Japan native controlled play from the baseline and never took his foot off the gas in a match that lasted just 67 minutes.  Wawrinka, who looked out of sorts at times, donated 31 unforced errors and could only counter with 12 winners in the match. Meanwhile, Nishikori capitalized on four out of his seven break point opportunities.

After dropping the first set, the world no. 4 attempted to pump himself up, but it was clear that he would not capture the level which earned him his third grand slam title in September.

“I felt very comfortable out there,” Nishikori said afterward. “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.”

With continued success in London, the 26-year old has an opportunity to finish the season at a career-high ranking of three.

Meanwhile, Wawrinka, who played with a wrap on his knee, acknowledged his struggles on the court, “I don't think I find anything on the court today. I was a little bit slow on everything. I was hesitating a lot with my game, my movement,” he said.

In the nightcap, new world no. 1 Murray cruised past Marin Cilic 6-3, 6-2 despite a subpar serving day.

The Scot overcame some sloppy play in a first set that included four double faults but Cilic could not take advantage.  Despite only two winners in the set Murray was successful on both service break opportunities while the Croat won just one out of his four chances to break serve.

In the second set, Murray significantly cleaned up his ground strokes although his serve continued to falter. He benefited from 13 unforced errors from Cilic while contributing only five and broke serve twice to seal the set and the match.

After his first match ranked world no. 1, Murray reflected, “The last few days, I haven't thought about it too much,” he said. “It didn't change much for me.”

He will face Nishikori on Tuesday while Cilic will square off against Wawrinka as play continues.