Nicolas Kiefer (born 5 July 1977), is a former German professional tennis player. He reached the semifinal of the 2006 Australian Open and won a Silver medal in Men's Doubles with partner Rainer Schüttler at the 2004 Summer Olympics in Athens. Kiefer's career-high singles ranking was World No. 4, achieved in January 2000.

Kiefer was taken notice of as an outstanding junior. He won the Junior Australian Open, US Open and was a finalist and semi-finalist at Wimbledon and the French Open finishing as the No. 2 junior behind Mariano Zabaleta when he was 18 in 1995.

On 10 January 2000, he reached his second quarter-final at the Australian Open and afterwards was ranked World No. 4, his highest position to date.

Nicolas has been known to have a few tennis superstitions. He is sometimes seen tapping his racquet on the corners of the court after a point, although the reasons behind this are not clear. He also, when serving, frequently asks for the ball with which he has just won a point to re-use it in the next one.

Kiefer became infamous for an incident on 25 January 2006, during the quarter-finals of the Australian Open. While facing Sébastien Grosjean late in the fifth set of a marathon match, Kiefer threw his racquet midpoint. Grosjean lost the point, hitting the ball into the net. Grosjean protested that the racquet distracted his shot. The umpire Carlos Bernardes said he did not believe the act was intentional and noted Grosjean had already hit the ball before the flying racquet could have had any effect on his shot. Grosjean eventually lost the fifth and final set to Kiefer. Kiefer went through to the semi-finals where he was defeated by the 2004 champion Roger Federer.

Kiefer injured his wrist while playing at the 2006 French Open, and announced his return on 5 July 2007, having fallen to the 404th position on ATP. He announced that he was "tired of waiting and anxious to start traveling again and to see his name on scoreboards". Kiefer returned at the 2007 Gerry Weber Open, losing in the first round to eventual champion Tomáš Berdych. At Wimbledon, he made the third round after defeating No.30 seed Filippo Volandri and Fabrice Santoro, both in straight sets, before losing in 4 sets (3 of which were tiebreakers) to Novak Djoković. At Newport, however, he ended up losing in round 1. At Los Angeles, he reached the semifinals in only his 4th tournament since coming back from injury; he had to default against Radek Štěpánek, another player coming back from injury, because of an injury sustained during his quarter-final win. He also made an impressive showing at the 2007 Madrid Masters, where he beat number five seed Fernando González in the quarterfinals before losing in the semifinals to world number one Roger Federer 6–4, 6–4.

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