Washington - Reigning US Open champion Marin Cilic and 2014 US Open runner-up Kei Nishikori advanced to the quarter-finals of the ATP and WTA Washington Open with straight-set triumphs Thursday.
Japanese second seed Nishikori, the highest-ranked player in the field after Britain's Andy Murray crashed out in his opening match, never faced a break point in dispatching Argentina's Leonardo Mayer 6-4, 6-4, after 92 minutes.
"I'm very confident and I'm feeling really good," Nishikori said.
Croatian third seed Cilic, who faced a possible rematch with Nishikori in the semi-finals, outfought 32nd-ranked American Sam Querrey 7-6 (7/2), 7-6 (7/3) at the US Open hardcourt warm-up event.
"I'm pretty confident about my game," Cilic said. "I'm 30 percent better at this moment than last year."
French fourth seed Richard Gasquet, coming off a Wimbledon semi-final run, was upset by 37th-ranked American Jack Sock 6-7 (4/7), 7-6 (7/5), 6-4 after two hours and 33 minutes.
"It took me a while to get into a rhythm," Sock said. "I knew if I stayed close and kept fighting I would have a chance."
Women's top seed Ekaterina Makarova of Russia also reached the last eight, defeating Britain's Naomi Broady 6-4, 7-6 (7/2). She next plays Romanian sixth seed Irina-Camelia Begu.
Nishikori, who seeks his third title of the year after Memphis and Barcelona and the 10th of his career, silenced speculation in Japan he might be becoming a father, saying, "I hope one day, but not yet."
Fifth-ranked Nishikori broke Mayer for a 4-3 edge and held twice more to take the first set in 59 minutes, then broke again in the penultimate game of the match when the South American netted a forehand volley after having saved two other break points.
"He was returning well. That's why I could only break him twice," Nishikori said.
The 25-year-old Asian number one will next face Australia's Sam Groth, who fired 20 aces to defeat Spanish seventh seed Feliciano Lopez 6-3, 6-4 in 59 minutes.
"I'm going to have to serve well, make him feel uncomfortable. I'm going to have to attack the net and move well," Groth said. "I'm going to play my style. I have nothing to lose."
Nishikori has never faced Groth, who is ranked a career-best 62nd this week.
"Never easy to play big servers," Nishikori said. "But I'm returning well. I'll concentrate on serving well and try to take advantage of my small chances."
Cilic improved to 4-0 all-time against Querrey, their prior matchup a 2012 Wimbledon epic third-rounder Cilic won 17-15 in the fifth set. Cilic rolled to a 6-1 edge in each tie-breaker to win.
"In the tie-breakers, I was very focused to put all the returns in and be as focused as possible to make him play as many strokes as possible," Cilic said. "The results came out with two comfortable tie-breakers but if one or two things go wrong it could have been pretty tight."
Blocking Cilic's path to his second semi-final of the year is Alexander Zverev, who rallied past Ukraine's Alexandr Dolgopolov 2-6, 6-2, 6-4. The 18-year-old German became the youngest Washington men's quarter-finalist since a 17-year-old Andy Roddick in 2000 by eliminating the 2012 Washington winner.
Next up for Sock is 55th-ranked countryman Steve Johnson, who dumped Bulgarian sixth seed Grigor Dimitrov 6-3, 1-6, 6-3. It's Washington's first all-US quarter-final since 2003.
Russian giant killer Teymuraz Gabashvili, who ousted Murray, was eliminated 7-5, 6-4 by Lithuania's Ricardas Berankis, who will next meet John Isner, the US eighth seed who defeated Canada's Vasek Pospisil 6-4, 7-6 (8/6).
Results on Thursday from the fourth day of the ATP and WTA Washington Open (x denotes seeding):
Kei Nishikori (JPN x2) bt Leonardo Mayer (ARG) 6-4, 6-4
Sam Groth (AUS) bt Feliciano Lopez (ESP x7) 6-3, 6-4
Marin Cilic (CRO x3) bt Sam Querrey (USA) 7-6 (7/2), 7-6 (7/3)
Alexander Zverev (GER) bt Alexandr Dolgopolov (UKR) 2-6, 6-2, 6-4
Steve Johnson (USA) bt Grigor Dimitrov (BUL x6) 6-3, 1-6, 6-3
Jack Sock (USA) bt Richard Gasquet (FRA x4) 6-7 (4/7), 7-6 (7/5), 6-4
Ricardas Berankis (LTU) bt Teymuraz Gabashvili (RUS) 7-5, 6-4
John Isner (USA x8) bt Vasek Pospisil (CAN) 6-4, 7-6 (8/6)
Anastasia Pavlyuchenkova (RUS) bt Belinda Bencic (SUI x3) 6-2, 6-4
Christina McHale (USA) bt Yulia Putintseva (KAZ) 6-2, 1-6, 7-5
Ekaterina Makarova (RUS x1) bt Naomi Broady (GBR) 6-4, 7-6 (7/2)