Introduction
Jaime Oncins (born June 16, 1970) is a former tennis player from Brazil.
Oncins represented his native country at the 1992 Summer Olympics in Barcelona, where he reached the quarterfinals before falling to Russia's Andrei Cherkasov. The right-hander won two individual career titles (Bologna and Búzios, both in 1992). He reached his highest singles ATP-ranking on May 3, 1993, when he became world no. 34.
Oncins' best performance at a major was at the 1992 French Open, where he reached the Round of 16, losing to eventual finalist, Petr Korda. In the second round of the tournament, Oncins had a famous victory from two sets down against former world no. 1 and three-time French Open champion, Ivan Lendl. Oncins was also the last player that Jimmy Connors beat at the US Open. Connors beat Oncins in straight sets in the first round of the 1992 US Open, which was on Connors' 40th birthday.
Oncins was a runner-up in the Roland Garros mixed doubles event in 2001, with Paola Suárez from Argentina.
Grand Slam mixed doubles finals (1)
Runners-up (1)
| Year | Championship | Partnering | Opponents in Final | Score/Final |
| 2001 | French Open |  Paola Suárez |  Tomás Carbonell Virginia Ruano Pascual | 7–5, 6–3 |
Career finals
Singles (2)
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| Outcome | No. | Date | Championship | Surface | Opponent in the final | Score in the final |
|---|---|---|---|---|---|---|
| Runner-up | 1. | October 28, 1991 | Búzios, Brazil | Hard | Jordi Arrese | 6–1, 4–6, 0–6 |
| Runner-up | 2. | November 4, 1991 | São Paulo, Brazil | Hard | Christian Miniussi | 6–2, 3–6, 4–6 |
| Winner | 1. | May 18, 1992 | Bologna, Italy | Clay |  Renzo Furlan | 6–2, 6–4 |
| Winner | 2. | November 2, 1992 | Búzios, Brazil | Hard |  Luis Herrera | 6–3, 6–2 |
| Runner-up | 3. | November 9, 1992 | São Paulo, Brazil | Hard |  Luiz Mattar | 1–6, 4–6 |
Doubles (5)
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| Outcome | No. | Date | Tournament | Surface | Partner | Opponents in the final | Score in the final |
|---|---|---|---|---|---|---|---|
| Runner-up | 1. | December 31, 1990 | Wellington, New Zealand | Hard |  John Letts | Luiz Mattar Nicolás Pereira | 6–4, 6–7, 2–6 |
| Runner-up | 2. | April 29, 1991 | Madrid, Spain | Clay | Luiz Mattar | Gustavo Luza Cássio Motta | 0–6, 5–7 |
| Runner-up | 3. | May 20, 1991 | Bologna, Italy | Clay | Luiz Mattar | Luke Jensen Laurie Warder | 4–6, 6–7 |
| Winner | 1. | November 4, 1991 | São Paulo, Brazil | Hard |  Andrés Gómez | Jorge Lozano Cássio Motta | 7–5, 6–4 |
| Winner | 2. | February 22, 1993 | Mexico City, Mexico | Clay | Leonardo Lavalle | Horacio de la Peña Jorge Lozano | 7–6, 6–4 |
| Runner-up | 4. | August 2, 1993 | Prague, Czech Republic | Clay | Jorge Lozano |  Hendrik Jan Davids Libor Pimek | 3–6, 6–7 |
| Winner | 3. | March 22, 1999 | Casablanca, Morocco | Clay | Fernando Meligeni | Massimo Ardinghi Vincenzo Santopadre | 6–2, 6–3 |
| Winner | 4. | June 7, 1999 | Merano, Italy | Clay | Lucas Arnold Ker |  Marc-Kevin Goellner Eric Taíno | 6–4, 7–6 |
| Winner | 5. | July 19, 1999 | Stuttgart, Germany | Clay | Daniel Orsanic |  Aleksandar Kitinov Jack Waite | 6–2, 6–1 |
| Runner-up | 5. | April 30, 2001 | Munich, Germany | Clay | Daniel Orsanic | Petr Luxa Radek Štěpánek | 7–5, 2–6, 6–7(5) |
| Runner-up | 6. | May 20, 2001 | Sankt Pölten, Austria | Clay | Daniel Orsanic | Petr Pála David Rikl | 3–6, 7–5, 5–7 |