(Reuters) - Defending champion Justin Thomas returns to action at this week’s CJ Cup in Las Vegas where four of the world’s top six players head an elite field warming up for next month’s Masters. The tournament is usually part of the PGA Tour’s Asian swing but was moved this year to Shadow Creek Golf Course from Nine Bridges in South Korea because of the COVID-19 pandemic. Initial favourite Dustin Johnson withdrew from the no-cut, 78-player event after a positive COVID-19 test but even without the world number one the tournament has a very strong field. Joining world number three Thomas at Shadow Creek are Spaniard Jon Rahm (two), Northern Irishman Rory McIlroy (four), PGA Championship winner Collin Morikawa (six) and Xander Schauffele (eight). Thomas, in his first start since earning a share of eighth place at last month’s U.S. Open, is hoping to increase his speed and distance over the next two weeks before he takes time off ahead of the Nov. 12-15 Masters. “The absolute No. 1 thing is I’m continuing to stay injury-free and I’m continuing to progress in a good direction of terms of staying healthy and staying fit,” said Thomas, who has won the event twice in its three-year existence. “But if I can do that while incorporating some more speed, then that’s big.” Four-times major champion Brooks Koepka, who was runner-up at last year’s Masters, makes his third return from an injury-related hiatus in 14 months and said he finally feels great. Koepka is confident his hip and knee issues are finally behind him and although he only started hitting balls about 10 days ago, he is not using this tournament as a way to ease himself back into competition. “Winning (is the goal). I know physically I’ll be able to walk four rounds and have no issue with it,” said world number 11 Koepka. “From there, it’s just go out and win.”