|Born||10 December 1975|
|Highest ranking||No. 569 (28 October 2002)|
|Highest ranking||No. 839 (28 October 2002)|
Couillard is a member of the Monegasque Davis Cup team since 2002, having posted a 14–10 record in singles and a 13–10 record in doubles in thirty-five ties played.
In 2013, Couillard and Benjamin Balleret played the longest professional tiebreak in known tennis history, lasting 70 points (34–36). Couillard lost the match 6–7(34), 1–6 in the third qualifying round for the USA F1 Futures in Plantation, Florida.
|Grand Slam (0–0)|
|Tennis Masters Cup (0–0)|
|ATP Masters Series (0–0)|
|ATP Tour (0–0)|
|Runner-up||1.||June 17, 2002||Kigali, Rwanda||Clay||Gwenael Gueit||0–2, RET.|
|Runner-up||2.||February 23, 2004||Benin, Nigeria||Hard||Valentin Sanon||6–1, 6–7, 6–7|
- Chase, Chris (2013). "The Longest Tiebreak in Tennis?", USA Today, 7 January 2013.
- Guillaume Couillard at the Association of Tennis Professionals
- Guillaume Couillard at the International Tennis Federation
- Guillaume Couillard at the Davis Cup