Li Na carved out another bit of grand slam history for China, then joked she would grab a bag of chips and enjoy watching Serena Williams take on Carla Suarez Navarro. That, though, is unlikely to have been pleasant viewing. After Li became the first Chinese player to reach the semi-finals at the US Open with a laboured 2½-hour win over 24th seed Ekaterina Makarova, she watched the seemingly unstoppable Williams crush her Spanish opponent 6-0, 6-0, the first "double bagel" in a quarter-final at Flushing Meadows since 1989, when 18-time major winner Martina Navratilova did it to Manuela Maleeva. Worryingly for Li, who has to stop Williams if she is to reach the final, the American top seed and defending champion insisted her game wasn't peaking and that she had plenty of room for improvement. Li beat Russia's Makarova 6-4, 6-7 (5-7), 6-2. On three occasions, Li went up a break in the second set, nearing eventual victory each time, but could not finish it off. She was then up 3-0 in the tiebreaker before faltering, but recovered well. Li has only won one of nine career matches against Williams heading in tomorrow's semi-final. "Tough, tough opponent," Li said. "But it's a good challenge to play against her." Li insisted she would try to focus only on herself. Having watched Williams' demolition job, that may prove tricky. "I mean, if you only think about what [your] opponent [is] doing, of course you already lose the match before you come to the court," Li said. "For tennis you have to figure out what you have to do on the court, what you should do." Williams is looking better and better with each match at the year's last grand slam tournament and Li will have to find new levels of play if she is to deny the American a place in the final. Williams is going for her fifth title at Flushing Meadows and 17th major overall, while Li became the first Asian grand slam winner with her French Open title in 2011. Williams won 53 of 71 points and dominated pretty much every statistical category. The first set took all of 19 minutes. The second was slower, lasting 33 minutes, but no less lopsided. Williams was asked whether she'd describe her performance as close to flawless. "Of course not," she laughed. "I played good, though. I played really good. I was just more focused than anything. You know, I like to believe there is room for improvement." Asked if her game was peaking, Williams replied: "No. Not yet. I hope not. I'm just trying to do the best that I can." Through five matches, Williams has dropped a total of 13 games so far. Going back to the start of Wimbledon last year, the 31-year-old American is 96-5 with 13 trophies, including from three of the past five grand slam tournaments plus the London Olympics. "She's the best player in the sport," Suarez Navarro said. "When you look at the draw, you don't want to see Serena there." Li said of her semi-final place: "I always try to be the first one. Of course it was exciting. I was really proud of myself because it's the last grand slam of the year, but I'm still fighting a lot on the court to try my best." She will have to fight harder than ever to become the first Chinese to reach the final.