Ken Flach, who won four Grand Slam titles in men's doubles and two in mixed doubles, has died, according to the ATP World Tour and the International Tennis Federation. He was 54.
The ATP World Tour announced on Tuesday that Mr. Flach died on Monday night in California after a brief illness.
In a post on Facebook, Mr. Flach's widow, Christina Friedman, wrote that he "passed away after fighting pneumonia."
"Ken was taken far too soon and his sudden passing comes as a real shock to everyone in tennis," ATP president Chris Kermode said, adding that Mr. Flach "will be remembered as one of the great U.S. doubles players in the history of our sport."
Mr. Flach was commemorated on Tuesday on the main stadium court at the BNP Paribas Open in Indian Wells, Calif. His picture was shown on the video board and the announcer mentioned his career highlights.
Mr. Flach reached No. 1 in the men's doubles rankings and paired with Robert Seguso to form one of the world's top teams in the 1980s.
They won 28 titles together, including major championships at the U.S. Open in 1985 and Wimbledon in 1987 and 1988. They twice were the runners-up at the U.S. Open.
Mr. Flach and Mr. Seguso also collected a gold medal for the United States at the 1988 Seoul Olympics.
They played together at Southern Illinois University at Edwardsville and won three Division II U.S. national championships, according to the ATP.
Mr. Flach finished with a total of 34 men's doubles titles, including winning the 1993 U.S. Open with Rick Leach. Mr. Flach's Grand Slam mixed-doubles trophies came with Kathy Jordan at the French Open and Wimbledon in 1986.
Mr. Flach played doubles on the U.S. Davis Cup team from 1985 to 1991, compiling an 11-2 record.
He retired as a professional tennis player in 1996 and became a coach, leading Vanderbilt University to the NCAA finals in 2003.